Vaadin 6: HTML5 video element with m4v - no sound

Hello everybody,

I am using the Video element and a m4v (external) resource.
The video is running - but without sound.

my video: H.264, AAC, AC3
mime type used and/or tested: video/mp4 video/x-m4v

ExternalResource r = new ExternalResource("test.m4v", "video/mp4");
Video video = new Video("video player test", r); 

browsers tested: safari (6.0.5) and firefox (22 and 23) on a mac (10.8.4).
vaadin: 6.8.12

Does somebody have a hint?

Thanks a lot!