When I load up my additional resource packs, even if it is only one, the Default pack still overrides the sound files, or plays them simultaneously. I use the Modern HD pack most often and the alternate tracks (which are amazing) are replaced or played alongside the original record tracks. Rock does not sound good with the original "ward" playing in the background, if it plays the alternate at all. This happens with both the Jukebox and world music.
I have had no crash problems so far, just some annoying sounds. That said, the textures are taking priority nicely. I have noticed nothing unusual about the appearance or functionality of any of the new blocks so far. Keep up the good work Mojang.