Tom, I was also gong to suggest one of those in line volume controls from rat
shack but the last time I got one it was some insane price like $15.
One thing you could do is change the frequency of your tone to one that wasn't
quite so annoying and/or so loud as both the human ear as well as the circuitry
in the rig and speaker will respond slightly differently to tones of different
frequencies. It won't make much deifference but if it doesn't distress you to
change sidetone frequency it might be enough and it'd be free.
Tom
Tom Brennan KD5VIJ, CCC-A/SLP
web page http://titan.sfasu.edu/~g_brennantg/sonicpage.html