It is fairly standard for echo bots to say something at the beginning of each audio call (“this is an echo test, anything you say will be echoed back to you”). This allows the user to check that he can receive packets from the bot and that his headphones are working correctly before starting the echo test. I know that the "!playsong" helps in this matter already.
A harder problem is to tell the difference between a dead microphone and packets not being received by the bot. It would be great if the bot could indicate, by chat (written), how many packets it is receiving a second and if they contain anything other than silence. If there are no packets being received, there is a network problem. If they are full of silence, there is an audio problem.
These proposed changes to the Google Talk voice echo debug number would make debugging network/audio problems easier.
Okay... I'm going to mark this as low priority. Sorry.
Microphone issues would be better debugged by using your system's volume meter. You could submit a feature request to your favourite VoIP client to have volume meters for the streams it's sending/receiving. I think that's a more appropriate place for it. I have opened a separate bug for having a text-based introductory message.