If sound works generally, but not inside Flash Player (in FireFox), try this:
thushara@agni:~$ sudo apt-get install libflashsupport
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
0 upgraded, 1 newly installed, 0 to remove and 131 not upgraded.
Need to get 8326B of archives.
After this operation, 65.5kB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com hardy/universe libflashsupport 1.9-0ubuntu1 [8326B]
Fetched 8326B in 0s (14.9kB/s)
Selecting previously deselected package libflashsupport.
(Reading database ... 99009 files and directories currently installed.)
Unpacking libflashsupport (from .../libflashsupport_1.9-0ubuntu1_i386.deb) ...
Setting up libflashsupport (1.9-0ubuntu1) ...
Processing triggers for libc6 ...
ldconfig deferred processing now taking place
restart FireFox and sound should be available...