1. open the message thread
2. hit the menu button
3. click the little paper clip that says "attach"
4. select "audio"
5. choose your ringtone and select "done"
I don't know the size limit that Handcent imposes on the audio files sent, but ringtones and notification tones I make with Ringdroid that are only a few seconds work fine (3 to 6 second files I have successfully sent; ringtones longer than that like 30 second clips of songs are too big apparently).
Sorry if this doesn't help, maybe I'll download ChompSMS to see how that works forwarding audio...