Try this:
Press the TP button (traffic report announcement thingy), keep the button pressed and then turn on the system (by pressing the big round button in the centre on my Concert). This then comes up with a 'TA' volume settings (twiddle the volume to desired level), press the TP button again and the display will then show you 'GALA', this is the volume sensitvity for speed (ie the volume will increase the faster you go) - again twiddle the volume to reach desired level, on mine its 0-5, 5 I presume being most active in its increasing of volume. Press the TP button again and it shows 'VOL' and THIS I believe is the setting for the 'turn-on' volume setting, so no matter how loud you had it last time, it'll only come-on at this desired level, again 0-5 the options.
Post back the outcome.