Like the OP, I have a zillion of my own MP3/iTunes stuff on my PC (well, NAS for me).
To me, Alexa is just a voice command system, it'll take a few more pieces.
Like most folks say, hook your Dot to a "real" speaker so it sounds better. Bluetooth or a cable to your old stereo would work. Unagumented Dots are good for rooms where you just want voice commands. Echo is for rooms where you want it to sound good (mileage may vary). Problem is Dots are cheap, so they're more likely given as presents to parents (such as yourself). Plus side is, they make good starters, and you can move them to secondary rooms as you expand your Alexa Empire.
Next getting Alexa access to those files.
I hear Amazon is phasing out their Drive system, so uploading MP3 to Amazon is both limited and might not stick around.
So, I got Plex. It's free (the free version is all you need for this). Install it on your PC with all the music. Let it find everything.
Now install the Plex skill on your Alexa and link it to your Plex account.
Then say "Alexa, ask Plex to play XYZ" where XYZ is something you own in your collection.
It is mostly that easy. Getting your files to Amazon would give you a more direct command "Alexa, play Fulsome City Blues" but if you've got a huge collection like I do (got a huge stack of CDs in my closet), it doesn't scale well.