First of all, I'm not very knowledgeable in this department so please bear with me and ask if something sounds odd.

I've a .m4a file supposed to contain a song (multichannel) from a DTS source. I have no trouble playing .m4a files in general or listening to DVDs with DTS sound. But when I try to play this file in iTunes (since it's an .m4a I thought it might work in Apples own program) and VLC I only get static. In foobar2k I don't even get static.

I've also tried to convert the file to a multichannel .wma, but I just got an error message during the process informing me that the file couldn't be converted (kind of expected I guess).

Can anyone help me out?

I'm not sure, but I think that lossy encoding like AAC would destroy DTS data, making it undecodable.

To reduce the number of variables you're testing (so to speak), decode the .m4a to .wav, and rename it with the .dts extention and try playing it in foobar. If it still doesn't play, I'd be willing to bet that your DTS was wrecked by lossy compression.