There are three things to consider:
- Storage of the music.
The music files have to be available to the Pi. This means setting up up a share on the Pc so the Pi can access the files and having them in the correct format.
- Connection to the stereo.
The Pi has very basic audio capabilities and I would look to one of the many many HATs that are available others are available). The majority of these use I2C to link to the Pi and provide a digital to analogue converter. You will need to find a HAT that outputs signals that are compatible with your stereo. Note Aux ports can be input (from an auxiliary device) or output (to an auxiliary amp and speaker set).
- Something to provide control
This piece of software has to reside on the Pi, link to the music, link to the HAT on the Pi and take control. Sounds a lot but it's a common need and a search for Pi music players will give you lots and lots. My personal choice is Volumio - I am not affiliated to them but a happy user but this takes over the Pi completely as it can:
- Link to remote sources of music
- Index the music (inc album art) without being concerned on the music format as long as the music meta data is present in the file
- Let you control it from a web browser on your PC or an app running on Android or IOS devices. I think the Android app handles multi-room sync'd output where as my iOS version does not. The apps are chargeable - this is one way they fund the development BUT web browsing on my iPhone is fine as the page is reactive to device size and orientation.
- Output via the HAT or to Bluetooth devices (inc cheap portable Bluetooth headphones and speakers)
Lots of alternatives are available (see here for some examples) but I would also think about:
- Is you output (i.e. amp and speakers) capable of decent music playback?
- Does your collection have the correct data esp album links?
- Would you be better just to get a small portable set-up that takes a SD card?
You can find a few videos about this by John Darko on YouTube He is an audio nut so some solutions are the cost of a small house but there are a few Pi based ones as well.
mount
the Samba or Windoze share from your RPi 3) connect RPi audio jack to AUX input on your audio system 4) SSH into RPi from Android & play music from the share. – Seamus Nov 14 '20 at 22:31