As far as I can tell, there are currently are 7 audio streams in Android:
STREAM_ALARM (for alarms) STREAM_DTMF (for DTMF Tones) STREAM_MUSIC
STREAM_MUSIC is the default in the AOSP source, defined in TextToSpeech.java (line 164 as of this writing) in frameworks/base.git:
STREAM_MUSIC
frameworks/base.git
/** * Default audio stream used when playing synthesized speech. */ public static final int DEFAULT_STREAM = AudioManager.STREAM_MUSIC;