Do you think removing /dev/mixer1 and maybe /dev/dsp1 would fix
this?
Would it be safe to delete these two devices considering I have only one
sound card?
I don't think it would hurt. Ideally only the devices you really have
should be represented in /dev. It won't fix the error though, just
remove the symptoms, which you've already done.
It might not even be an error. It might be some program probing to see
how many sound devices you have.
Björn Persson