You will find what you are looking for listed as a USB DAC (Digital to Analog Converter) even though technically, toslink is not analog and you may not need the DAC functionality for your purpose.
There is a long list on amazon. This one claims to be Plug-and-play and need no drivers.
Part of that which is missing in today's portable communications devices (cellular phones) is a feature of old-time telephones known as sidetone. This is exactly what you describe as feedback. One can hear one's own voice when speaking, with minimal to zero delay. Any appreciable delay becomes comedy material for radio disk jockeys and is riotous.
I did a ...
6) How about a 3.5mm male-3.5mm female four pole TRRS (tip-ring-ring-sleeve) extension cord?
The four pole is needed since it's a headset, not just headphones (which only need three pole, tip-ring-sleeve). Examples shown at https://www.google.com/search?psb=1&tbm=shop&q=3.5mm%20headset%20extension%20cable .
Please confirm the plug on the end of ...
Standalone pitch adjuster devices like the TC_Helicon which can adjust pitch to the accuracy you specify are studio devices, designed to fit into a 19" rack mount frame. You'd need to interface them between the audio source and your amplifier, if your amplifier supports that. They will all be more expensive than used PC running tone adjustment apps.
If you can't find such a cable you can replace it with two:
Those products are just an example.
As a bonus without the splitter you can use this as a headset with your mobile phone.
The way I've solved this problem in the past is with the PC's line-in jack. While it requires the PC to have power to output sound, it allows you to chain the output. You may need to go into the sound device manager for your PC's operating system and toggle the Line-In jack to get this to work, but you should only have to do that once. Some computers ...
The other answer's suggestion is a bit expensive for my taste (with several hundred dollar).
So I can recommend the soundcard I'm using myself to power my 5.1 speaker system. However you don't really need all the features so instead I'm gonna recommend the functionally largely equivalent, but cheaper version:
The ASUS Xonar U5
On Amazon.com and on the ...
The focusrite 2i2 has what you need. Here is a link that has the full specs, but some of the relevant ones are:
2x line/mic/instrument combination inputs high quality XLR / 1/4" TRS Jack
Combo 2x Line/Instrument switches
2x Gain knobs
2x Gain halo signal indicators 48V Phantom power switch Direct monitor switch
Large monitor level dial