Which OS are you using? And can you give more info on the type of speakers/system etc. But here are a few things to try but will vary depending on if you use Vista or XP...
Go to Start > Control Panel > Sound and see if there are any devices there. If so, click configure and see if that does anything (probably won't but worth checking). Then click Properties and go through the tabs to see if there is anything that has been disabled/switched off. Notably, check if the audio out is disabled or not!
If that fails, then you may need to install/update your sound card driver in which case you need to go to Control Panel > Devices and under the sound and game controllers check to see if you Sound Card device is there. If so, right click > Properties > Driver tab and then select update driver. PM if you have any issue and I can run through it over MSN or something if you want!