how to select default sound device in alsa?

How do I set the default device?
Create a configuration file (either for the whole system at /etc/asound.conf or .asoundrc in your home directory) containing statements like the following ones:

defaults.ctl.card 1
defaults.pcm.card 1
defaults.timer.card 1

This file would make card 1 the default. The card number (1 in this example) is taken from the /proc/asound/cards file.

NOTE: use the number given by alsamixer - F6/S for a default snd device.

cause: usb cam/microphone was added to pc which messed up the sound device order.