After downloading the Shockwave Flash 9 Player from:

http://www.adobe.com/go/getflashplayer

and installing the plugin with:

tar xvzf install_flash_player_9_linux.tar.gz
cd install_flash_player_9_linux
./flashplayer-installer

Copyright(C) 2002-2006 Adobe Macromedia Software LLC.  All rights reserved.
Adobe Flash Player 9 for Linux
Adobe Flash Player 9 will be installed on this machine.
You are running the Adobe Flash Player installer as the "root" user.
Adobe Flash Player 9 will be installed system-wide.
Support is available at http://www.adobe.com/support/flashplayer/
To install Adobe Flash Player 9 now, press ENTER.
To cancel the installation at any time, press Control-C.
NOTE: Please exit any browsers you may have running.
Press ENTER to continue...
Please enter the installation path of the Mozilla, Netscape,
or Opera browser (i.e., /usr/lib/mozilla): /usr/lib/firefox
----------- Install Action Summary -----------
Adobe Flash Player 9 will be installed in the following directory:
Browser installation directory = /usr/lib/firefox
Proceed with the installation? (y/n/q): y
Installation complete.
Perform another installation? (y/n): n
Please log out of this session and log in for the changes to take effect.
The Adobe Flash Player installation is complete.

When navigating to a website with flash content that has sound Firefox will freeze, and the application has to be terminated. For example:

http://www.adobe.com/shockwave/welcome/

Audio device:

#lspci | grep -i audio
00:1e.2 Multimedia audio controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) AC'97 Audio Controller (rev 03)

Install alsa-oss to use the aoss command. Aoss is a wrapper script to allow non-supported applications to force using ALSA:

apt-get install alsa-oss

Save the following as /etc/asound.conf

pcm.!default {
    type plug
    slave.pcm "dmixer"
}

pcm.dsp0 {
    type plug
    slave.pcm "dmixer"
}

pcm.dmixer  {
    type dmix
    ipc_key 1024
    slave {
        pcm {
            type hw
            card 0
        }
        period_time 0
        period_size 1024
        buffer_size 8192
        rate 44100
    }
    bindings {
        0 0
        1 1
    }
}

The following entries in /etc/firefox/firefoxrc may also resolve some issues:

FIREFOX_DSP="none"
export XLIB_SKIP_ARGB_VISUALS=1

Further Information

CentOS 4.2/4.3/4.4 on ThinkPad T43
http://www.cs.umd.edu/~nakamura/miscellaneous/t43_centos4.html#alsa