Greg,
Per the SL30 Installation Manual:
Sidetone Level Adjustment
The sidetone volume was preset at the factory to a typical audio level. The level can be adjusted using one of the built-in system functions. To adjust the sidetone level:
1. Press SYS, rotate the large knob to the COM RADIO INFO: page, and press ENT.
2. Rotate the large know to the SIDETONE LEVEL: page.
3. Rotate the small knob to adjust the sidetone level. The sidetone level is displayed in a range of 1 to 100, and variable. The sidetone level can be adjusted during transmit.