[Trunk] Blank sound default in Mixer for scores that don't play that instrument

1. Open score including piano and vocals.
2. Play.
3. Go to Synthesiser.
4. Load soundfont with one instrument (piano).
(Result: Voice isn't heard)
5. Open Mixer.
(Result: Soundfont is listed as voice in sound, despite not being heard).
6. Click on the soundfont.

Result: Soundfont is now heard.

Discussion: I think by default (until selected), there should be a blank option that outputs no sound (see 1.0 behaviour for one such solution).

Using MuseScore 2.0 Nightly Build (4367) - Mac 10.6.7.