At this moment in time mkv files with dual audio can only properly play the first track in the VU+ stb.

If the audio you wish to play is not the first track then the VU+ will struggle to play it without glitches.

One method to fix this is to use mkvmerge from the freeware MKVtoolnix application to remove the other audio tracks from the mkv file and thus create the mkv file with soley the audio track you want.

Run "mmg.exe" , this is the gui front-end for the mkvmerge command line program.

Add your mkv file with multiple audio. Select an output path to save the altered file.

Untick the tracks you wish to remove.

In the example above:
A_MPEG/L3(ID1,type: audio) is an MPeg layer 3 (MP3) audio track that I wish to remove - untick this
V_MPEG4/ISO/AVC(ID 2,type: video) is the picture part of the movie
A_AAC(ID3,type: audio) is the AAC encoded audio track I wish to keep

When you are satisfied that there is only 1 audio tack and 1 video track ticked in the list click START MUXING

The output file will now have only 1 audio track and should play properly in the VU+

Thanks Mikes