I don't have my Nexus yet, so I can't make sure if this works. Go in MXplayer and when playing an MKV file click the sound button on the top right (its probably a musical note) and choose the software decoding option. Let me know if this works. If...
I had the app working on my phone but when I tried activating the Google Prepaid Card, it would say that it was verifying my account with Money Network and it got stuck there. It was a while ago though and I'm trying to recreate it.