Lately, IMMERSE releases have had problems, as well as many other release groups. The "no audio" issue is not the only symptom I have noticed lately. I have noticed that tsMuxer.exe, ffmpeg.exe have all had issues with MKV playback, displaying errors more often than usual.
More blackscreens with tsMuxer, or sound streams in a file not playing....etc. etc. or slow motion video.
I believe, though, that the cause is generally the same, an issue with muxxing or writing.
You could try unchecking "force FPS parsed from ffmpeg in the meta file" under tsMuxer tab in PMS.
Might help with these particular files????
Or you could try some of the fixes suggested in this link.viewtopic.php?f=6&t=7203&start=0
Or, if you don't lose too much quality, and your CPU can keep up, just run the MEncoder option. Generally, the IMMERSE tv releases are not too high bitrate, and MEncoder should work, especially as 720 releases.