Using ; in an extension list is widely supported.
1. ext:.mp3;.mp4;.wav
You can use a . in your extension list, including the . may make it more confusing.
2. ext:mp3|mp4|wav (should be a difference between a | without a leading space and <space>|)
I'll consider supporting | in extension lists too, we could get away with this because ext is an OR list and a following OR would be unlikely. A space could be used to break the term, where | would normally break the term. The Everything term parser would need to know that ext: eats | before splitting up terms.
Adding a special case just for ext: may also cause confusion, the user might expect | to break the term as it does with every other term.
I will add support for: ext:"mp3|jpg"
Double quotes always escape operators.
3. ext:<mp3|mp4|wav>
Using < > could also work, I'll consider it in a future release, it's more to type. The Everything term parser would need to know that ext: eats < | and >.
datemodified:jan;feb;mar
You must specify a single date or date range, for example:
datemodified:jan-mar
If you want multiple dates, you will need to use multiple ORed terms:
datemodified:jan | datemodified:mar