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.