DD716TED wrote:
But I want to use the computers internal built in speakers as well as an amplifier connected to the audio line out jack.
I would guess that all computers work the same way. At least on my desktop, 2 laptops, and tablet. The audio is either in OR out.
The switchover is mechanical, not electrical. The act of plugging in changes the audio output from in to out.
On one of my laptops, when pushing in a headphone plug, there is a place where BOTH will operate at the same time. The audio output socket has mechanical contacts that are "make before break". In other words, the audio outputs are BOTH made (make) before one (inside) is break (switched over).
Using an external speaker or headphones, SLOWLY push in the plug and see if there is a point where both will be on at the same time.
It works for me.
Good luck.
Tim