Added volume controle on sample level.

libao-async will adjust the volume of the samples.

libao.rkt

@@ -18,6 +18,8 @@
          ao-bufsize-async
          ao-clear-async
          ao-pause
+         ao-set-volume!
+         ao-volume
          ao-valid?
          ao-valid-bits?
          ao-valid-rate?
@@ -177,7 +179,20 @@
   (ffi:ao_bufsize_async (ao-handle-async-player handle))
   )
 
-(define (ao-clear-async handle)
+(rc:define/contract (ao-set-volume! handle percentage)
+  (rc:-> ao-handle? number? void?)
+  (ffi:ao_set_volume_async (ao-handle-async-player handle)
+                           (if (integer? percentage)
+                               (exact->inexact percentage)
+                               percentage))
+  )
+
+(rc:define/contract (ao-volume handle)
+  (rc:-> ao-handle? number?)
+  (ffi:ao_volume_async (ao-handle-async-player handle))
+  )
+
+(rc:define/contract (ao-clear-async handle)
   (rc:-> ao-handle? void?)
   (ffi:ao_clear_async (ao-handle-async-player handle))
   )