no sound

Radio on internet? Which site you using to listen to radio? Using inpage flash based radios or using streaming audio in Amarok or any such app?

I just have installed ubuntu, and don't know what kind of appl to use with it. pls tell me what to install.

no sound playing CD.
Yesterday it was working.

Navigate to "System > Administration > Software Sources". Under the first three tabs (Ubuntu Software, Third-Party Software, and Updates) enable all repositories but 'proposed' and 'backports'. Leave "Automatic Updates" untouched also.

Then run

sudo wget http://www.medibuntu.org/sources.list.d/hardy.list -O /etc/apt/sources.list.d/medibuntu.list && sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update && sudo apt-get autoremove totem-mozilla && sudo apt-get install ubuntu-restricted-extras mozilla-mplayer libdvdcss2 w32codecs

from a terminal window.
You are now sorted for mp3 and DVD playback as well as flash and video web content.

still no sound, after execution.

Assuming that you ran the above properly and error-free, you are set for media playback on the codec side of things.

Can you check in alsamixer whether nothing's muted?
--> MM is muted, OO represents an open channel

Further, please post the output of

cat /proc/asound/cards

and

sudo lshw -C multimedia

momchil@momchil-desktop:~$ cat /proc/asound/cards
 0 [Live ]: EMU10K1 - SB Live 5.1 Dell OEM [SB0220]
                      SB Live 5.1 Dell OEM [SB0220] (rev.10, serial:0x80661102) at 0x9000, irq 19
 1 [V8237 ]: VIA8237 - VIA 8237
                      VIA 8237 with ALC655 at 0xc400, irq 20
momchil@momchil-desktop:~$ sudo lshw -C multimedia
[sudo] password for momchil:
  *-multimedia:0
       description: Multimedia audio controller
       product: SB Live! EMU10k1
       vendor: Creative Labs
       physical id: a
       bus info: pci@0000:00:0a.0
       version: 0a
       width: 32 bits
       clock: 33MHz
       capabilities: pm bus_master cap_list
       configuration: driver=EMU10K1_Audigy latency=32 maxlatency=20 mingnt=2 module=snd_emu10k1
  *-multimedia:1
       description: Multimedia video controller
       product: CX23880/1/2/3 PCI Video and Audio Decoder
       vendor: Conexant
       physical id: c
       bus info: pci@0000:00:0c.0
       version: 05
       width: 32 bits
       clock: 33MHz
       capabilities: vpd pm bus_master cap_list
       configuration: driver=cx8800 latency=32 maxlatency=55 mingnt=20 module=cx8800
  *-multimedia:2
       description: Multimedia audio controller
       product: VT8233/A/8235/8237 AC97 Audio Controller
       vendor: VIA Technologies, Inc.
       physical id: 11.5
       bus info: pci@0000:00:11.5
       version: 60
       width: 32 bits
       clock: 33MHz
       capabilities: pm cap_list
       configuration: driver=VIA 82xx Audio latency=0 module=snd_via82xx
momchil@momchil-desktop:~$

thanks!
the sound preferences were set to Autodetect.
That was the problem.

Thanks Johannes Pilkahn, that solved my question.