Problem conserving 5.1 from MKV conversion

[l'ensemble]

Hello all--just a quick question, maybe someone can help me:

I've been converting files in mkv format (with 5.1 audio) using visualhub's "itunes" "Optimize for appletv 5.1, 2.0" setting for a while and have had no problems--they convert to .mov files and contain both 5.1 and 2.0 audio feeds. However, as of last week, visualhub keeps converting them to an .mp4 format, which still retains the original hd video but none of these contain the 5.1 audio (only 2.0). So, anything it converts to an mp4 loses the 5.1 and is stereo only, while it's mov converts retain it. Any reason why this may be? [I've alternatively tried using handbrake, but that also converts to 2 channel, even though it's supposedly preserving the 5.1 and using trials of Aunsoft mkvconverter, those convert to "multichannel" format, not 5.1 dolby digital, as preserved in the mkv container).

Any ideas on how to get visualhub converting properly to the dual format again?

thanks!

[l'ensemble]

P.S. I haven't changed any of the original settings of visualhub either, so nothing there.

P.P.S. Not sure if it matters, but I'm using a 1.82 ghz dual core mac mini running osx 10.5 (leopard) and the latest/last version of visual hub (I think 1.34).