diff --git a/libao-async-ffi.rkt b/libao-async-ffi.rkt
index 983c2ca..6734c35 100644
--- a/libao-async-ffi.rkt
+++ b/libao-async-ffi.rkt
@@ -21,8 +21,8 @@
 (define _BufferType_t
   (_enum '(ao = 1
            flac = 2
-           mpg123 = 3
-           ao_ogg = 4
+           mp3 = 3
+           ogg = 4
            )))
 
 ;#define AO_FMT_LITTLE 1
@@ -63,12 +63,15 @@
 ;extern void ao_stop_async(void *handle);
 (define-libao-async ao_stop_async(_fun _libao-async-handle-pointer -> _void))
 
-;extern void ao_play_async(void *handle, double at_second, double music_duration, int buf_size, void *mem, BufferInfo_t info);
-(define-libao-async ao_play_async(_fun _libao-async-handle-pointer _double _double _uint32 _pointer _BufferInfo_t -> _void))
+;extern void ao_play_async(void *handle, int music_id, double at_second, double music_duration, int buf_size, void *mem, BufferInfo_t info);
+(define-libao-async ao_play_async(_fun _libao-async-handle-pointer _integer _double _double _uint32 _pointer _BufferInfo_t -> _void))
 
 ;extern double ao_is_at_second_async(void *handle);
 (define-libao-async ao_is_at_second_async(_fun _libao-async-handle-pointer -> _double))
 
+;extern int ao_is_at_music_id_async(void *handle);
+(define-libao-async ao_is_at_music_id_async (_fun _libao-async-handle-pointer -> _integer))
+
 ;extern double ao_music_duration_async(void *handle);
 (define-libao-async ao_music_duration_async(_fun _libao-async-handle-pointer -> _double))
 
diff --git a/libao.rkt b/libao.rkt
index b92d610..f4f0af7 100644
--- a/libao.rkt
+++ b/libao.rkt
@@ -6,6 +6,7 @@
          ffi/unsafe/custodian
          data/queue
          "private/utils.rkt"
+         (prefix-in rc: racket/contract)
          )
 
 (provide ao-open-live
@@ -13,10 +14,17 @@
          ao-close
          ao-at-second
          ao-music-duration
+         ao-at-music-id
          ao-bufsize-async
          ao-clear-async
          ao-pause
-         ao-valid? 
+         ao-valid?
+         ao-valid-bits?
+         ao-valid-rate?
+         ao-valid-channels?
+         ao-valid-format?
+         ao-handle?
+         ao-supported-music-format?
          )
 
 (define device-number 1)
@@ -34,10 +42,42 @@
   )
 
 
+(define (ao-supported-music-format? f)
+  (and (symbol? f)
+       (or (eq? f 'flac)
+           (eq? f 'mp3)
+           (eq? f 'ao))))
+
+
 (define (bytes-for-bits bits)
   (/ bits 8))
 
-(define (ao-open-live bits rate channels byte-format)
+(define (ao-valid-bits? bits)
+  (and (integer? bits) (or
+                        (= bits 8)
+                        (= bits 16)
+                        (= bits 24)
+                        (= bits 32))
+       )
+  )
+
+(define (ao-valid-rate? rate)
+  (and (integer? rate)
+       (not (eq? (memq rate '(8000 11025 16000 22050 44100
+                                   48000 88200 96000 1764000
+                                   192000 352800 384000)) #f))))
+
+(define (ao-valid-channels? c)
+  (and (integer? c)
+       (>= 1 c)))
+
+(define (ao-valid-format? f)
+  (or (eq? f 'little-endian)
+      (eq? f 'big-endian)
+      (eq? f 'native-endian)))
+
+(rc:define/contract (ao-open-live bits rate channels byte-format)
+  (rc:-> ao-valid-bits? ao-valid-rate? ao-valid-channels? ao-valid-format? ao-handle?)
   (let ((handle (make-ao-handle device-number)))
 
     (fin:register-finalizer handle
@@ -69,18 +109,27 @@
     )
   )
 
-(define (ao-close handle)
-  (unless (eq? (ao-handle-async-player handle) #f)
-    (info-sound "ao-close - closing handle")
-    (ffi:ao_stop_async (ao-handle-async-player handle))
-    (set-ao-handle-async-player! handle #f)
-    )
+(rc:define/contract (ao-close handle)
+  (rc:-> ao-handle? void?)
+  (void
+   (unless (eq? (ao-handle-async-player handle) #f)
+     (info-sound "ao-close - closing handle")
+     (ffi:ao_stop_async (ao-handle-async-player handle))
+     (set-ao-handle-async-player! handle #f)
+     )
+   )
   )
 
 (define (ao-valid? handle)
-  (not (eq? (ao-handle-async-player handle) #f)))
+  (and (ao-handle? handle)
+       (not (eq? (ao-handle-async-player handle) #f)))
+  )
 
-(define (ao-play handle at-time-in-s music-duration-s buffer buf-len buf-type)
+(define (any? x)
+  #t)
+
+(rc:define/contract (ao-play handle music-id at-time-in-s music-duration-s buffer buf-len buf-type)
+  (rc:-> ao-handle? integer? number? number? any? integer? ao-supported-music-format? void?)                    
   (let* ((bytes-per-sample (ao-handle-bytes-per-sample handle))
          (bits (ao-handle-bits handle))
          (rate (ao-handle-rate handle))
@@ -88,37 +137,48 @@
          (endianess (ao-handle-byte-format handle))
          (buf-info (ffi:make-BufferInfo_t buf-type bits rate channels endianess))
          )
-    ;(dbg-sound "ao-play ~a ~a" at-time-in-s music-duration-s)
     (unless (ao-valid? handle)
       (err-sound "Cannot play on an invalid ao-device")
       (error "Cannot play on an invalid ao-device"))
     (ffi:ao_play_async (ao-handle-async-player handle)
+                       music-id
                        (exact->inexact at-time-in-s)
                        (exact->inexact music-duration-s)
                        buf-len
                        buffer
                        buf-info)
+    
     )
   )
 
-(define (ao-pause handle pause)
+(rc:define/contract (ao-pause handle pause)
+  (rc:-> ao-handle? boolean? void?)
   (dbg-sound "ao-pause ~a" pause)
   (ffi:ao_pause_async (ao-handle-async-player handle) (if (eq? pause #f) 0 1))
   )
 
-(define (ao-at-second handle)
+(rc:define/contract (ao-at-second handle)
+  (rc:-> ao-handle? number?)
   (ffi:ao_is_at_second_async (ao-handle-async-player handle))
   )
+
+(rc:define/contract (ao-at-music-id handle)
+  (rc:-> ao-handle? integer?)
+  (ffi:ao_is_at_music_id_async (ao-handle-async-player handle))
+  )
   
-(define (ao-music-duration handle)
+(rc:define/contract (ao-music-duration handle)
+  (rc:-> ao-handle? number?)
   (ffi:ao_music_duration_async (ao-handle-async-player handle))
   )
 
-(define (ao-bufsize-async handle)
+(rc:define/contract (ao-bufsize-async handle)
+  (rc:-> ao-handle? integer?)                    
   (ffi:ao_bufsize_async (ao-handle-async-player handle))
   )
 
 (define (ao-clear-async handle)
+  (rc:-> ao-handle? void?)
   (ffi:ao_clear_async (ao-handle-async-player handle))
   )