iPod touch G2 external mic (not headphones) how to activate the speaker?

Just bought an external mic and the t-pain app, the blurb said "mic headphones recommended" (note required) but when I plug the mic in, it cuts the external speaker. How can I re-activate the iPod touch speaker so I can hear the beat while recording?

(And there's no way I'm going to spend £30+ on a headphone mic just to have a few hours fun with one app before anyone asks!)

Use a headphone splitter, works for me.