Now I grabbed the latest (stable?) 1.3.1 from Aminet for another test. The first good thing is that it uses "native" monitor drivers again. I'm not a big fan of those multiscan/VGA drivers. Also, the sound is working well again (maybe I messed up my settings when I tested the last beta). It IS more stable now for sure and the the sound might be more accurate too, although it is a bit slower than the previous 1.2.1. But games like Day Of The Tentacle and Sam & Max are still playable and Full Throttles's intro and action sequences are still slow as hell.
One little issue which also appeared in 1.2.1 (I think I've mentioned it somewhere before): I usually set up the audio via the global configuration. When I start a game and return to the launcher again the override mode is switched on and of course uses different settings, because it wasn't configured yet. Is that normal?