You didn't specify whether you're just seeing "aU" in the terminal, or if you're having audio playback issues. I'm going to assume it's the later.
This will help with the audio playback
create this file if it does not exist, otherwise edit the file: ~/.gnuradio/config.conf
Add this section to the file if your configuration uses ALSA. It's Ubuntu, it probably does...
[audio_alsa]
period_time = 0.100 nperiods = 5
I don't know what the hell is up with the OP25 repository/wiki. This place seems just as dead...
There are a few forks on github:
mmhhpp/op25 https://github.com/mmhhpp/op25 balint256/op25 https://github.com/balint256/op25 bitmunch/op25 https://github.com/bitmunch/op25 << this has a performance update thanks to Luke, and a few other minor tweaks and stuff