Create null sound port only once and reuse it

Ignore-this: 883f6ccdca2a3a7cd074748275b74b19

darcs-hash:20130918135329-56b4d-64ee6288e308edebea1b2558035e0cb44bc6fc14
Simplified audio device fallback code

Ignore-this: 488f4ed83a1e8663c3b96ecdda745f4e

darcs-hash:20130918141412-56b4d-604d1019192a72fd262362aa831342c01412ce17
Reordered methods of AudioMixer class

Ignore-this: 9209b8565bca4895408282cf63bcd646

darcs-hash:20130918141940-56b4d-fbafcb0e8419d2b29cdd79d2216f73c844e3cb95