Annoying click every 2 seconds, high CPU usage, ... when running on battery power

I noticed alsa-tray causes an annoying click roughly every 2 seconds when my laptop is running on battery power. It also uses far more CPU time (peaks of 100% usage right before every click). The program reacts extremely slowly to any change in volume, and the slider is almost unusable.

I think some kind of power management system is repeatedly switching the sound card on and off. I tried replacing (almost) all temporary mixer objects with one persistent object, so the mixer isn't opened and closed all the time, and this seems to fix the problem.

Note: the line numbers in the diff file may not be correct, because I already made some other changes to fix another bug:
https://bugs.launchpad.net/alsa-tray/+bug/970767