There ARE some workarounds for the loud turn signals. As crpediem_2 has shared, there are gain settings in the DCM menu under the Signal Management tab. He showed the standard gain adjustment, which is a single, global adjustment for all inputs. If you choose the Advanced Gain Setup, gain can be adjusted for three different input pairings (Front, Rear and Subwoofer) independently. The turn signal is contained in the FRONT L channel so increasing the gain of the Front R&L pair (or increasing the global gain) will increase the turn signal level. So one trick is to leave the Front Input gain at a lower level.
I stumbled onto this after receiving a tune from Bimmertech, explaining that I had a Match UP8 DSP connected with a Bimmertech wiring harness and upgraded Bimmertech Alpha One subwoofers. The tune was very much to my liking as it seemed to have a more "diffuse" sound wasn't so front center-focused. The tune also reduced the output gains for the Front L and Center speakers to help lower the turn signal sound. I tweaked it a bit to my own tastes and like it better than my attempts at creating an all-original tune using the UMIK-1 microphone and the RTA settings within the PC-Tools software.
If anyone is interested in trying the tune out, please have a go. Just delete the ".pdf" from the end of the filename.