Is there a way to apply an effect to multiple sequences at once? For example, I have 50 or so sequences, and I need to universally change the audio levels on all of them. Is there a fast way to do this other than by doing individually per sequence?
You can adjust a single clip, make your adjustment the copy that clip. Use butterfly-A to select all and option-V to PASTE ATTRIBUTES. Check audio levels and hit OK. All clips will now have that value. If your copied clip was at -2dB all clips will be at -2dB. They will not be at whatever value they had previously -2dB.
If you're wanting to adjust sequences instead of clips on sequences nest all of your sequences onto a timeline, then treat them as clips would have been treated in the advice above.
Finally, the audio mixer (option-6) has a master fader. To be honest I don't think I've ever used it, but it's there.
You can also open each sequence, select all, then press cmd+opt+l and adjust all relatively or absolutely. Only problem here is that if some clips are already maxed out at +12, and you bring up other clips, then you lose the relative mix. The master fader is a better option. It's setting does affect output to quicktime and everything. It is adjusted per sequence as well. I use it all the time for master volume in the suite. I use roland powered speakers, so it's a hassle to adjust their volume since each speaker has it's own knob.