To rule out driver and software issues, you may try a live Linux disc (Ubuntu or something like that)
My son opened the laptop and he said that the onboard sound cannot be fixed.
Opening the laptop was not a good idea since now any warranty is void.
If there is really something wrong with the audio hardware, then using an USB audio device together with external speakers or headphones are your only option then. You can't use the internal speakers with an external audio device.