Codec: Realtek ALC888 - Mixer: Generic 10de ID 3 - ALC888 Analog [ALC888 Analog] Audio device: Intel Corporation 82801I (ICH9 Family) HD Audio Controller (rev 03) - sound works via speakers, but not via headphones on Acer Aspire 5738G in Ubuntu 9.04 - using ALSA driver 1.0.18rc3

Try opening a terminal (Applications > Accessories > Terminal" and executing "alsamixer".
It is a terminal mixer that shows volume for each device.
You'll see a lot of vertical bars, each one labeled with the device it controls.
The first should be "Master", which is the Master volume control for your system, and the second (or the third) should be "Headphones".
If the volume is off, the bar will be completely black; raise it by pressing your Up key.
If the volume is muted, you will see "MM" under the bar; press "m" to unmute.

Here's an explaining image: http://yfrog.com/0dalsamixerbp

You can notice how the master volume is at 64/100, the headphones at 100/100, the speaker at 0/100 but not unmuted and the S/PDIF is muted.
Hope this helped :)

Well I tried doing what you suggested.. But the headphones audio was 00 and would'nt go up either..
Moreover, even if I plugin headphones, the audio comes through laptop speakers.. Furthermore, I think there is a drivers problem also.. In System Settings>>multimedia, it says there are no drivers for HDA Nvidea(Conexanent analog)/HDA Nvidea(Conexanent Digital). But I have no clue how to update these devices...

Please send us the full terminal output from step 3 and step 4 from this procedure:

https://help.ubuntu.com/community/SoundTroubleshootingProcedure

Please also specify the exact model and make of your PC (if possible) on this thread

Which computer do you have?

My computer name is Acer Aspire 5738G. Intel t6400 core 2 duo. 512mb Nvidia graphics card. any other specific details?

Please also send us the full terminal output from step 3 and step 4 from this procedure:

https://help.ubuntu.com/community/SoundTroubleshootingProcedure

My ALSA information is located at http://pastebin.ca/1791307

Please try this procedure:

Step 1: Open Terminal from "Applications->Accessories->
Terminal"

Step 2: Run the following command (copy/paste command into the Terminal and then hit <enter>)

gksudo gedit /etc/modprobe.d/alsa-base.conf

In the gedit editor, scroll down to the last lines of the alsa-base.conf file and add these 2 lines to the end of the alsa-base.conf file (with the file open inside the gedit editor!):

alias snd-card-0 snd-hda-intel
options snd-hda-intel model=auto

Step 3: Reboot pc and retest sound via headphones.

Step 4: If sound is still not working after a reboot, replace the auto model option with one of the options below (for example: replace model=auto with model=3stack-dig or model=acer-aspire) and then reboot and retest sound again.

Between reboots, only test 1 model option at a time!

132 ALC883/888
133 ==========
134 3stack-dig 3-jack with SPDIF I/O
135 6stack-dig 6-jack digital with SPDIF I/O
136 3stack-6ch 3-jack 6-channel
137 3stack-6ch-dig 3-jack 6-channel with SPDIF I/O
138 6stack-dig-demo 6-jack digital for Intel demo board
139 acer Acer laptops (Travelmate 3012WTMi, Aspire 5600, etc)
140 acer-aspire Acer Aspire 9810
141 acer-aspire-4930g Acer Aspire 4930G
142 acer-aspire-6530g Acer Aspire 6530G
143 acer-aspire-8930g Acer Aspire 8930G
144 medion Medion Laptops
145 medion-md2 Medion MD2
146 targa-dig Targa/MSI
147 targa-2ch-dig Targa/MSI with 2-channel
148 targa-8ch-dig Targa/MSI with 8-channel (MSI GX620)
149 laptop-eapd 3-jack with SPDIF I/O and EAPD (Clevo M540JE, M550JE)
150 lenovo-101e Lenovo 101E
151 lenovo-nb0763 Lenovo NB0763
152 lenovo-ms7195-dig Lenovo MS7195
153 lenovo-sky Lenovo Sky
154 haier-w66 Haier W66
155 3stack-hp HP machines with 3stack (Lucknow, Samba boards)

156 6stack-dell Dell machines with 6stack (Inspiron 530)

157 mitac Mitac 8252D
158 clevo-m720 Clevo M720 laptop series
159 fujitsu-pi2515 Fujitsu AMILO Pi2515
160 fujitsu-xa3530 Fujitsu AMILO XA3530
161 3stack-6ch-intel Intel DG33* boards
162 asus-p5q ASUS P5Q-EM boards
163 mb31 MacBook 3,1
164 sony-vaio-tt Sony VAIO TT
165 auto auto-config reading BIOS (defaul

Try muting the front channels and UNmuting the surround channel to get the headphones to work.

Actually the previous procedure solved the problem.. Thanks a million.. I am amazed at the speed of help that i have recieved from you.
Everything is working perfectly great.
If I knew I would have happily treated you to a free lunch... :-)
Thanks once again..
Yours Sincierly
Suhas
India

You're welcome.