Johannes,
It looks like you're doing everything right, so I'm not sure why it's not working for you.
I just set up a similar test, capturing a folder on an external volume, excluding all of the *.dmg files, and it worked as advertised.
So here are my suggestions:
Make sure the path is correct If you're typing in the path, it's easy to mess it up. Instead, select the path portion of the pattern, clear its contents, and then locate the folder in the Finder and drag that into the path field (the insertion cursor must be active for this to work). That will alway paste in the exact path to the folder.
Make sure the extension is correct Select one of your .mp3 files, choose Get Info in the Finder, then look at the Name & Extension section. Make sure the extension is correct and it's really the extension of the file. (Some files have what look like extensions, but their extension is hidden in the Finder and the "extension" you see is actually part of its name.)
Verify where you are creating the pattern You didn't mention whether you were setting this pattern in the Archive's settings or in the Capture Preferences of an item. When using the capture preferences, absolute paths are not allowed and if you create a pattern with an absolute path, it will be ignored. So you have the choice of adding the pattern to the archive's settings or using a relative path in a pattern attached to a folder:
Archive Settings: path=/Volumes/WorkingMedia/Sound room
Capture prefs for /Volumes/WorkingMedia/Sound room: path=(empty)
Capture prefs for /Volumes/WorkingMedia: path=Sound room
Finally, you also have the option of creating a
.qrecall_exclude_patterns.txt file in the "Sound room" folder. It would consist of a single, simple, repeating, glob pattern:
//*.mp3/+
Let us know if any of that helps.