Following a post at Ubuntu Forums I’ve managed to fix my flash sound (which has been broken for about 3 months). The bit that did it in the end was creating an /etc/asound.conf file (as root) with the following content.

Note that I have an ES1370 soundcard like the author of the original post. YMMV.

pcm.!default {
type plug
slave.pcm “dmixer”
}
pcm.dsp0 {
type plug
slave.pcm “dmixer”
}
pcm.dmixer {
type dmix
ipc_key 1024
slave {
pcm “hw:0,0”
period_time 0
period_size 1024
buffer_size 8192
rate 44100
}
bindings {
0 0
1 1
}
}

ctl.dmixer {
type hw
card 0
}

Woohoo! Looks like it’s fixed and I can now watch YouTube vids on my old Athlon 1.1GHz HP Pavilion 7880 (UK) computer with sound. Thanks to themeone.