I don't remember if something like this ever happened here. Go to settings and increase default Anti-click time or/and change mode to a diff one. If you are there you can also decrase amount of channels, should help in quality.
Thanks for your reply. Unfortunately I already decreased the number of channels. That didn't help.
Also I tried increasing the anti-click time, but that also didn't help.
I tried to change every possible setting that is available, but nothing works.
Only if I use the workaround with AmigaAmp I have no problems at all with mplayer or OWB's youtube html5 player.
Right now, AHI music unit is set to unit 0, and the ahi music unit 0-3 are set to the sam's onboard audio.
AmigaAmp is set to use unit 0.
Anyway, it works now. It would be nice if there was a more "clean" solution, but it's better then no fix at all.