finally a working libao-async racket backend

libao.rkt

@@ -1,7 +1,7 @@
 #lang racket/base
 
 (require (prefix-in fin: finalizer)
-         (prefix-in ffi: "libao-async-ffi.rkt")
+         (prefix-in ffi: "libao-async-ffi-racket.rkt")
          ffi/unsafe
          ffi/unsafe/custodian
          data/queue
@@ -164,13 +164,14 @@
     (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)
+    (void 
+     (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))
     
     )
   )