This one still needs working on. I've set up both the original Nexus 7 and the newer version of that device and I have to say... the mono speaker totally sucks. For some music, it's kind of a little bearable but when playing movies and games the volume is so low and hallow it's disgusting. You have to use either a headset or external speakers. I just bought a Kindle Fire HDX to give as a gift so I have no idea how those speakers sound.
Tried coupling it with Chromecast? I found this point to be most pertinent with Chromecast and Plex.
Not sure what you're doing wrong, but the 2013 Nexus 7 has two speakers. You definitely shouldn't still be getting mono.