[Trunk] Changed Mixer settings not aurally retained after viewing other scores

1. Open attached scores.
2. Open 'Second Score'.
3. Open 'Mixer'.
4. Change these: Volume - Full. Pan - Left. Reverb - None. Chorus: None.
5. Open 'First Score'.
6. Play.
7. Click on the other score.
8. Play.

Result: Changes to Mixer settings haven't been retained aurally (visually, they're fine).

Discussion: If you change the settings of each, the sound will correct. Pan does not, however.

Using MuseScore 2.0 Nightly Build (5323) - Mac 10.6.8.