It is hard to guess, what iGO compilation you have installed on your device, what skins are there, etc. Different skins have these controls on different places. However, to give you a straightforward solution and if you don't mind to loose all your iGO settings (ie. reset iGO to default), you can erase iGO config files. To do so, locate your iGO directory on your memory card and delete Save folder. This will cause that after new iGO start this folder will be re-created, you will be prompted to define your language, voice and other custom settings, etc. again, like at the beginning and all iGO settings will reset to their defaults. With a bit of luck, also the sound level will get fixed.
As I said, this seems to be quite radical solution, but in your case this is the most straightforward.