There was no sound from the start.
After that I begin experimenting (I don't know was it necessary, or sound from all AppVMs should work as is).
I created a StandaloneVM (with the command "qvm-create mystandvm --standalone --label orange").
In the Qubes VM Manager I clicked "Show/Hide inactive VMs".
Then I select mystandvm and choose "VM settings" from context menu.
On the "Devices" tab I moved "00:1b.0 Audio device: Intel Corporation 82801JI (ICH10 Family) HD Audio Controller" to the right list.
Then I started VM with "Start/Resume VM".
After notification about successfull start I choose Firefox and went to youtube.com.
I tried to view something from xterm, it shows
[root@mystandvm ~]# lspci
00:00.0 Audio device: Intel Corporation 82801JI (ICH10 Family) HD Audio Controller
alsamixer shows that the Master (and all other levels) are set to their maximums.
There is no sound in the movies from youtube.