If you want to specify multiple tags, you still use a single string, but the tags are separated by spaces, e.g. "foo bar". You can use an expression to generate such a string, but it must be a valid expression. The expression str(Midi.NoteNumber - 23) & " audio"
will do this as it will return something like "50 audio" - note the space before " audio". Using a number is an odd kind of tag though, I'd suggest something like "note" & str(Midi.NoteNumber - 23) & " audio"
, which will produce a tag string like "note50 audio", i.e. two tags "note50" and "audio".