When I try to open one of these voice mails, K-9 mail pops up a very specific error message: "Unable to find viewer for audio/x-wav".
I see that you're accepting audio/* here:
http://git.videolan.org/?p=vlc-ports/an ... b=HEAD#l82
But mysteriously, this happens in the logcat:
I/ActivityManager( 1302): START {act=android.intent.action.VIEW dat=content://com.fsck.k9.attachmentprovider/86c81c6e-c276-4ea1-930d-9e448f739c18/4908/VIEW typ=audio/x-wav flg=0x80001} from pid 21325
E/k9 (21325): Could not display attachment of type audio/x-wav
E/k9 (21325): android.content.ActivityNotFoundException: No Activity found to handle Intent { act=android.intent.action.VIEW dat=content://com.fsck.k9.attachmentprovider/86c81c6e-c276-4ea1-930d-9e448f739c18/4908/VIEW typ=audio/x-wav flg=0x80001 }
E/k9 (21325): at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1512)
E/k9 (21325): at android.app.Instrumentation.execStartActivity(Instrumentation.java:1384)
E/k9 (21325): at android.app.Activity.startActivityForResult(Activity.java:3190)
E/k9 (21325): at android.app.Activity.startActivity(Activity.java:3297)
E/k9 (21325): at com.fsck.k9.view.AttachmentView.showFile(AttachmentView.java:294)
E/k9 (21325): at com.fsck.k9.activity.MessageView$Listener$7.run(MessageView.java:1101)
E/k9 (21325): at android.os.Handler.handleCallback(Handler.java:605)
E/k9 (21325): at android.os.Handler.dispatchMessage(Handler.java:92)
E/k9 (21325): at android.os.Looper.loop(Looper.java:137)
E/k9 (21325): at android.app.ActivityThread.main(ActivityThread.java:4575)
E/k9 (21325): at java.lang.reflect.Method.invokeNative(Native Method)
E/k9 (21325): at java.lang.reflect.Method.invoke(Method.java:511)
E/k9 (21325): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
E/k9 (21325): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
E/k9 (21325): at dalvik.system.NativeStart.main(Native Method)