Binaural beats are useless on when played in speakers. This feature should automatically get disabled for internal speaker mode and selected Bluetooth devices (eg. bt speakers disabled, bt stereo headset enabled, etc)
Best implementation would be a black/whitelist for audio devices
Update after: BETA 20160203
Disabled on device’s speaker 
Enabled on stereo bluetooth headset
Enabled on bluetooth mono headset or stereo speakers <- wrong
An option to enable binaural sounds only for specific bluetooth devices would be awesome.
Copied from: https://urbandroid.uservoice.com/forums/264867-sleep-as-android/suggestions/11324607-make-binaural-beats-activation-conditional
See also Binaural To Speaker