I found the same problem with my bose in-ear headphones...I found a fix over at the xda website
"HKLM\Software\HTC\AudioManager_Eng\Config\music_p l ayer_volumn --> 20 (default: 100)
HKLM\Software\HTC\AudioManager_Eng\Information\vol umn --> 20 (default: 100)"
I change the values even lower too 10 because then I can turn it up higher more gradually... here's the original post...
http://forum.xda-developers.com/show...6&postcount=18
fyi this only effects the headphones I think, not the speakerphone...