sndlib/mixer.cpp:496:61: runtime error: left shift of negative value -1
sndlib/mixer.cpp:333:74: runtime error: left shift of negative value -1
sndlib/mixer.cpp:331:115: runtime error: left shift of negative value -2
x<<y is UB for y<0 or x<0. Using a multiply side-steps this (and compilers
optimize multiplies to the same ASM anyway).