asterisk users - Jul 2004 - Voicemail plays back at very low volume - how to make it louder?

Hello,

I've just set up Asterisk with a TDM400P (specifically, a 'TDM22B
bundle' - 2 FXS and 2 FXO, although I'm only using one of each
interface at the moment, in case that's relevant)

The problem I'm getting is that when I listen to a voicemail
message, the recorded message is played back at extremely low
volume. All the supporting prompts are at the correct volume,
it's just the incoming recorded message that is played back
almost inaudibly quiet.

Looking through the archives I see this type of issue has come up
before, but I don't see a solution. Am I missing something really
obvious?

Thanks

Bob