Thank you very much for the answer, but I want to avoid streaming as it would induce a delay of a couple of seconds already, and anyway, I suppose I could do what you're telling me in an easier way with the phonestreamer app and then delay the incoming on the other device.
I'm looking for a solution in the phone, with just the phone. If I had two devices I could always just connect a jack between the N900 and the other thing and then delay the audio with VLC, which is what I already do when I'm at home, so as I'm telling you, what I'm looking for is a solution using only the N900.
Anyway, thank you again!