Comment 7 for bug 343153

For the audio device problem, see this: http://mixxx.org/wiki/doku.php/troubleshooting#no_or_too_few_sound_cards_appear_in_the_preferences_dialog_-_how_can_i_fix_this

Yes, the waveforms are CPU-intensive by nature.

You may need to delete your library file (mixxxtrack.xml) since Mixxx won't try to re-detect BPMs that are already listed in there unless you explicitly do so by right-clicking a track. I have noticed that the BPM detection in 1.7.0 is noticeably better than in 1.6.1 (it picks up BPMs on tracks it couldn't before.) The only problem is that the range might be too restricted depending on your music, so try opening it up: Go to Preferences->BPM Detection and mark the "Allow BPM above the range" box. Let us know if that helps.