Public bug reported:

User A opens a sound producing application, uses the computer, and
finishes, locking the screen. User B comes along and switches to their
user. User B can still hear user A's sound producing application. When
user B tries to play their own sound, they can't. Either they get a
dummy output device or everything appears to work, but they don't hear
anything.

How to reproduce:
Login to first user.
Open a song with rhythmbox or totem, and set the player to repeat mode so you 
wont get to the end of the song before you're done.
Switch to second user.
Open the Sound Preferences window (You can get to it at System > Preferences > 
Sound)
Go to the output tab and note the device. (Either you will have the normal 
device, or a dummy device)
Go to the applications tab.
Open Rhythmbox and try to play a song. (Either everything will appear to work, 
but you will hear nothing except the first user's sound, or you will not be 
able to get Rhythmbox to start playing.)

Why this is a problem:
Many flash-based web games have a sound output. If user leaves this open, even 
with the sound muted in the game, and then locks their screen and a second user 
comes along and logs in, wanting to use sound, they won't be able to. The only 
way they can fix the problem is either run "sudo killall npveiwer.bin" (which 
means they need administrator rights) or restart the computer (losing the other 
user's session and still requiring administrative rights). This makes Ubuntu 
have bad multi - user support as far as sound is concerned.

Also, I didn't have this problem in Jaunty, users couldn't stop other
users from using sound.

bj...@zorion:~$ apt-cache policy pulseaudio 
pulseaudio:
  Installed: 1:0.9.22~0.9.21+stable-queue-32-g8478-0ubuntu11
  Candidate: 1:0.9.22~0.9.21+stable-queue-32-g8478-0ubuntu11
  Version table:
 *** 1:0.9.22~0.9.21+stable-queue-32-g8478-0ubuntu11 0
        500 http://us.archive.ubuntu.com lucid/main Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
AlsaVersion: Advanced Linux Sound Architecture Driver Version 1.0.21.
Architecture: amd64
AudioDevicesInUse:
 USER        PID ACCESS COMMAND
 /dev/snd/controlC0:  bjorn      1580 F.... pulseaudio
 /dev/snd/pcmC0D0c:   bjorn      1580 F...m pulseaudio
 /dev/snd/controlC1:  bjorn      1580 F.... pulseaudio
 /dev/snd/pcmC1D0p:   bjorn      1580 F...m pulseaudio
Card0.Amixer.info:
 Card hw:0 'Intel'/'HDA Intel at 0xdcff8000 irq 22'
   Mixer name   : 'Realtek ALC662 rev1'
   Components   : 'HDA:10ec0662,10438290,00100101'
   Controls      : 36
   Simple ctrls  : 19
Card1.Amixer.info:
 Card hw:1 'default'/'C-Media USB Audio Device    at usb-0000:00:1d.1-1, full 
speed'
   Mixer name   : 'USB Mixer'
   Components   : 'USB0d8c:0008'
   Controls      : 5
   Simple ctrls  : 3
Date: Sat Feb 27 23:58:41 2010
DistroRelease: Ubuntu 10.04
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Alpha amd64 (20100224.1)
NonfreeKernelModules: nvidia
Package: alsa-base 1.0.22.1+dfsg-0ubuntu3
PackageArchitecture: all
ProcEnviron:
 LANG=C
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.32-14.20-generic
SelectedCard: 1 default USB-Audio - C-Media USB Audio Device
SourcePackage: alsa-driver
Symptom: audio
Title: [USB-Audio - C-Media USB Audio Device   ] Playback problem
Uname: Linux 2.6.32-14-generic x86_64

** Affects: alsa-driver (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug lucid

-- 
User A can prevent User B from using sound applications by leaving a sound 
producing application open
https://bugs.launchpad.net/bugs/529372
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