I play the audio using snd_play & snd_loop.
Audio quality is 11kHz or 22kHz, mono, 8 bit.

When playing the game on my PC (with a sound card), no problem.

When playing the game on PC without sound card, the speaker make a noticible noice. But if I pause the game (using game_freeze), the noice gone. The PC that don't has the sound card use on board Gigabyte Realtek HD Audio. It should be good enough for my tiny audio right?

Any idea? How many max audio should be play on PC without sound cards?

Try this
doppler_factor = 0; // switch off the doppler effect

Ops... sorry. The problem comes again. So I think it has nothing to do with doppler. Maybe I mess up something else.

i have onboard sound card too but i never had such problem. try maybe installing new drivers for sound...

Yeah you are right :p The PC that I test didn't install so many driver properly. It's not my PC so I didn't check it until you hint me. Thank you.