I don't know if this is related, but maybe...do you have an .asoundrc file in your home directory in arch? that tells alsa where to send the output. If it is used for audio input as well, maybe that is it.
My version of .asoundrc for my usb speakers has 3 lines, in case this might help:
$this->bbcode_second_pass_code('', '
defaults.ctl.card 1
defaults.pcm.card 1
defaults.pcm.device 0
')
I know there is a way to find out which numbers a device is assigned to, but don't have my original notes any more.
When the speakers are not attached, the 1's in the above file must be zeroes, so if your .asoundrc has all zeroes, maybe that is what is wrong?