On some machine, after installing Ubuntu (7.04 Feisty and 7.10 Gutsy) no sounds coming out. By default, Ubuntu install Alsa as audio driver. This should be no problem for many sound card. This means, your audio should work perfectly.
But if your ubuntu box still silent, you may try this.

Check your sound preference: “System => Preferences => Sound”

Select your device, then try to “Test”. If still no sound, go to next step.

1. Open “Volume Control” by double click on the volume icon on tray.
Then go to “Edit => Preferences”

2. On “Preferences” window, check on “Surround” option

3. Back to “Volume Control” window, make sure “Surround” volume is not muted. And set the volume for this.

Now your Ubuntu box will speak loudly..