[Dyerryjex]

I actually use this as a focus for meditation, and have found the following:
Sometimes the sound has turned into music, that I listen to while I look at something, be it an astral landscape or hypnagogic imagery. Sometimes the music has stayed with me throughout the experience and I have actually come out of it actually listening to the music, and wishing I could recall it any time I chose, because it's usually 'heavenly', when it happens.
Sometimes, as I go deeper into trance it turns into voices. This is not to be feared- it is not an indication of schizophrenia, as voice-hearers hear them at non-trance times.
Most often it goes away as soon as I start to detach from the physical body (have an OBE into the RTZ) so it's also a good indicator of trance, at least at first.
After some time of meditating and becoming aware of it, it stops being a good indicator of OBE readiness, as it starts to occur in light trance.