User A can prevent User B from using sound applications by leaving a sound producing application open

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.

bjorn@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