I have a audio jack cable i.e.

and I want to listen to music on phone through computer speakers. It works, but.....

In the sound control panel settings>recording tab>line in properties>Listen tab, there is a tick box 'Listen to this device'.

When I tick it, I get a "good" sound, however their is a slight noticeable echo.

When I leave it un-ticked, I still get sound but it is not as good, however it has no echo.

I am assuming, when it is unticked, the sound is playing through the inner speaker that is connected to the motherboard (that plays beeps on error at pc boot). When I leave it ticked, the sound is playing through that inner speaker but also through the computer external speakers, however one is playing the sound "faster" than the other and that it why there is an echo (one is playing at a small time interval behind the other). Would I be right in saying this?

My question is, if this is true, is there a way to disable that motherboard speaker without opening up the pc and disconnecting it? I want to do this so that I can get what seems to be better sound from the external speakers only.

From checking up, you could do something in windows 7, go to device manager, view hidden devices and a device group 'Non plug and play devices' would appear

see How can I disable / enable the internal speaker?

However, apparently this hidden option has been disabled in windows 8, see third bullet point in following link

Features and options removed in Windows 8 - MSFN Forum

Would any have any ideas bar opening up the pc?