Basically I can play all audio, from system startup, but after using Hydrogen drum app, I can no longer playback any audio, or videos. Any help?
Doing this worked for me. In Hydrogen Drum Machine go to Tools -> Preferences. Then select Audio System. The first option has Auto, Jack, Alsa and PortAudio in the drop down menu, select PortAudio here. Click on Ok. Again click Ok when asked to restart Audio drivers.
I needed to restart Hydrogen once when the audio had crashed/frozen and it was fine afterwards.