Use AudioEngine instead of SimpleAudioEngine
AudioEngine::play2d() returns an audio ID you can use with AudioEngine::setVolume() to target a specific audio source.
Alternatively you could extend SimpleAudioEngine to support this feature. However it would require implementing it for each platform manually. For example for ios (CocosDenshion.h/cpp)
alSourcef(source, AL_GAIN, gain) could be used.
Edit: AudioEngine is still under the 'experimental' namespace and should be used with care.