Added async synchronization with current alsa volume level. Volume changes are posted async. Volume sliders are hidden if default volume indicator is hidden. Volume sliders are shown if default volume indicator is shown. Extension volume indicator is hidden if default volume indicator is visible. Extension volume indicator is shown if default volume indicator is hidden.

You need to disconnect the signal you connect in enable(). You should also hide your menu item to avoid showing two volume sliders in the pulseaudio case (probably easiest to add a separate function for that and call it both outside and inside the signal handler). (Also the readVolume() function is a bit sloppy as you assume that you only need a single read to get at the data you want - still, I guess that's OK with the buffer size you're setting) Rejecting due to the lingering signal connection, which will result in warnings after disabling the extension.

How can I do async single-reads on a file?

I'm not aware of any method to do that - the usual approach is to read repeatedly until EOF is reached (as you already do in the newer version)