Ok I came up with an idea: Hear a recording/voice say that "You're dreaming." during REM

Basically, you fall asleep with headphones on, or a music player ready (after WBTB). You need to play the sound/music when you are in REM, so it has to be programmed to play by that time, or playing no sound (until into REM).

I thought of having a voice (recording) say "Hey [dreamers name], I think you're dreaming! Why not do a reality check?" to induce a lucid dream. Hopefully, at the right volume this recording can become part of your dream, as you might find someone who actually walks up to you and says this. If not at the right volume, you might wake up to it. If not at the right time, you might not hear it at all.

There's the problem. About the volume: Does anyone know (scientifically if possible, in decibels) what volume would perfectly suit this, so this won't wake you up, but so you can hear it in a dream? I know that's a silly question (how would you measure?), so can anyone help with this, in any way?