The command in Lxp's reply above is a great solution if you liked everything about the Flat View approach other than having to click more than one button to get to it.
Assuming you want to use that, here are a couple of small changes to improve a couple of things I ran into while checking it out myself:
If there are lots of non-wav files in your folders, then you can speed things up slightly by applying the filter before switching on Flat View, so all the non-wav files never get added instead of being added and then removed. (I think this requires a fairly recent version of Opus 12 and didn't work older verisons.)
Set QUICKFILTER=*.wav command acts as a toggle if the pattern is already
*.wav, but we can prevent that with a couple of
This has both:
For the next version of Opus, we've also made a change so the two
@if:... lines can be replaced by
notoggle: at the start of the pattern, to make life easier, and make the command consistent with similar ones.
This command doesn't work yet, but will in the next Opus version:
Finally, if you want a button to return things to normal with a single click: