In the SID there is a register which allows you to change the global volume of the output in 4 bit precision = 16 steps, ranging from $0 to $f. If you're using a defMON tune in some other program you may want to control this register, to fade music in or out.
To do this, just change the code below and change the operand of the ora instruction. At least in some versions of the defMON player, this is found at location $10b1 in RAM. Make sure that the upper nibble is always zero. In essence, set this location to $0x, where x is a value between $0 (silent) and $f (max volume). Looking under the hood, the code looks like this:
.sidf_mode_gvol1 = *+1 lda #0 .sidf_mode_gvol2 = *+1 ora #$0f sta sidbase+$18 ;$D418/54296/SID+24 Select Filter Mode and Volume
.sidf_mode_gvol1 is modified by the player, and .sidf_mode_gvol2 can be modified by you.