This is how I set up my desktop system to play sounds from various applications simultanously. Namely these applications are XMMS, GAIM, MPlayer and Firefox (with Flashplugin). All working on my Intel onboard soundcard:

From /proc/pci:

Bus  0, device  31, function  5:
  Multimedia audio controller: Intel Corp. 82801DB (ICH4) AC'97 Audio Controller (rev 2).
    IRQ 9.
    I/O at 0xa800 [0xa8ff].
    I/O at 0xa400 [0xa43f].
    Non-prefetchable 32 bit memory at 0xe4000000 [0xe40001ff].
    Non-prefetchable 32 bit memory at 0xe3800000 [0xe38000ff].

I use the new 2.6 Kernel on my Desktop System which comes with Alsa. Compiling is straight forward here are the options I used for the soundcard (only selected ones shown):

Device Drivers  --->
  Sound  --->
    <*> Sound card support
      Advanced Linux Sound Architecture  --->
        <M> Advanced Linux Sound Architecture
        <M> Sequencer support
        <M>   Sequencer dummy client
        <M> OSS Mixer API
        <M> OSS PCM (digital audio) API
        [*] OSS Sequencer API
        <M> RTC Timer support
            Generic devices  --->
               <M> Virtual MIDI soundcard
               <M> Generic MPU-401 UART driver
            PCI devices  --->
               <M> Intel i8x0/MX440, SiS 7012; Ali 5455; NForce Audio; AMD768/8111

I load the modules in my /etc/modules file:

snd-intel8x0
snd-pcm-oss
snd-seq-oss

For using alsa you should install some tools, too.

#> apt-get install alsa-utils alsa-oss

ALSA comes with a method to mix multiple sound streams which is called dmix. ALSA-enabled applications should be able to all access the device at the same time when it is set up. Do this you'll need to set up an .asoundrc file in your home directory. I followed the descriptions at http://alsa.opensrc.org/index.php?page=asym to do so.

This is what my .asoundrc looks like:

#asym fun start here. we define one pcm device called "dmixed"
pcm.dmixed {
    ipc_key 1025
    type dmix
    slave {
        pcm "hw:0,0"
        period_time 0
        period_size 1024
        buffer_size 8192
        rate 48000
    }
}

#one called "dsnooped" for capturing
pcm.dsnooped {
    ipc_key 1026
    type dsnoop
    slave.pcm "hw:0,0"
}

#and this is the real magic
pcm.asymed {
    type asym
    playback.pcm "dmixed"
    capture.pcm "dsnooped"
}

#a quick plug plugin for above device to do the converting magic. saves
#typing when settng the pcm name in an alsa app
pcm.pasymed {
    type plug
    slave.pcm "asymed"
}

#a ctl device to keep xmms happy
ctl.pasymed {
    type hw
    card 0
}

#here we try to point the aoss script to our asymed device
pcm.dsp0 {
    type plug
    slave.pcm "asymed"
}

ctl.mixer0 {
    type hw
    card 0
}

#this sets the default device
pcm.!default {
    type plug
    slave.pcm "asymed"
}

It's pretty much the same as decribed in the given link. However I did not work very reliable for me. XMMS with using the ALSA output plugin did play a half song fine but then suddenly stopped playing without reason. Another problem was getting applications to work with it which doesn't support ALSA. The documents say to use a library wrapper called aoss. This tool maps OSS system calls to the dsp0 device to the asymed ALSA device. This did work for mpg321 but not for Firefox which simply froze when playing a flash with sound.

The solution to the above mentioned problems is to use a sound daemon. the only one I found working as expected is ARTS.

#> apt-get install arts

To give it the needed permission for realtime scheduling make it's startwrapper SUID root:

#> dpkg-statoverride --add --update root root 4755 /usr/bin/artswrapper

Arts needs to be started on login. Either do it from your ~/.xsession file or (if you using a standards compatible desktop environment like XFCE) put a symlink in your Autostart directory:

$> mkdir -p ~/Desktop/Autostart
$> ln -s /usr/bin/artswrapper ~/Desktop/Autostart/artswrapper

To make firefox use the ARTS daemon we use a library wrapper similar to the above mentioned aoss the only difference is it works It's called artsdsp - so for running Firefox with ARTS just run the following command from the Firefox directory.

$> artsdsp ./firefox

To be sure it works install the Flash Plugin and try my silly animation at http://splitbrain.org/Study/Media_Creation/index.php - if you don't hear sound then something is wrong.

Note: If you're using a debian package of Firefox you can set the soundwrapper in /etc/firefox/firefoxrc like this:

# which /dev/dsp wrapper to use
FIREFOX_DSP="artsdsp"

Easy - just install the plugin:

#> apt-get install xmms-arts

More easy - go to the preferences and select Arts at the Sound Method dropdown.

Well the documentation says there is an arts driver however my MPlayer doesn't have it (I us a precompiled Debian package from Marillat). mplayer -ao help gave me the follwing options:

Available audio output drivers:
 mpegpes	Mpeg-PES audio output
 oss	OSS/ioctl audio output
 alsa9	ALSA-0.9.x audio output
 esd	EsounD audio output
 sdl	SDLlib audio output
 null	Null audio output
 pcm	RAW PCM/WAVE file writer audio output
 plugin	Plugin audio output

Now the dmix feature configured above comes in handy again. We just use the asymed device which is able to mix multiple sources. So it's possible to have both - ARTS with multiple Soundsources and MPlayer - accessing the same ALSA device.

Just add this to the ~/.mplayer/config

  ao=alsa9:asymed            #or ao=alsa1x ...
  srate=48000