I don’t know what has happened, but suddenly my laptop can’t play any sound and the volume bar has gone.
It’s an Acer Aspire 4937 G, running Windows Vista Home Premium.
Restarting my laptop has fixed the problem (for now).
But I would like to know what caused this to happen in the first place, so that if it happens again I can fix it.
It may happen that the playback device is disabled in the list of devices.
May be you had a piece of software that did that.