The MT-32 music in Cannon Fodder does not work in DOSBox when cycles are too high; e.g. the music works at 8000 cycles but not at 10000 cycles. So, on a real system you may need to slow things down: turn off turbo mode (if applicable), disable CPU cache, use slowdown utilities, etc.

If i slow down the computer will the game still works well or does it slow down too?

Minimum processor requirement for Cannon Fodder is 20 MHz 386SX, so just be careful not to go below that level of speed.