Speakers don't have to be expensive to sound good in the TJ's otherwise horrific acoustic environment. But they do need to have a >90 dB Sensitivity level so they won't need more power than typical head units like our Sonys can put out. A very good speaker choice that meets that Sensitivity requirement (and not many speakers do) without being stupid-expensive is the Polk DB series. 6.5" Polk DBs will fit into the soundbar without modification, at least they did in my '97 TJ's sound bar.
For the front, the TJ's speaker size is a 4x6 which is not a good shape to make a good sounding speaker. It'd be better to convert that 4x6 opening to a round opening for a 5.25" speaker. There are plenty of sheet metal adapters to do that in a TJ like at
https://www.amazon.com/dp/B007ZDCGVG/?tag=wranglerorg-20
So far as an MP3 player goes that will plug into your AUX jack, I use an Apple iPod Nano player which you can find used for a good price on eBay or similar sites. Here are some on Amazon...
https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias=aps&field-keywords=apple+ipod+nano&tag=wranglerorg-20. Or just use your phone to store & play them though they aren't as easily controlled by a head unit as a Nano or similar MP3 player is.
I have a ton of CDs on mine and it does a good job, it even displays the album cover if you download the music to it properly. I loaded my CDs onto a program on my computer called Media Monkey which does a good job downloading onto players like the Apple NANO. And I don't know about your Sony but mine has built-in controls that perfectly control the Nano, even displaying the names, album, and genre on the Sony's display. Whoops your Sony needs a USB port for that, you'd have to control the MP3 player (like the Nano) manually if you just plug it into the Sony's Aux port.
Anyway, that combination has worked well for me for 10+ years, hope that helps.
Edit: P.S. If you buy one of the newer Sonys with built-in Bluetooth, you can make a wireless connection to your Android or iPhone for really easy playing. Though one thing I noticed about my Android's Bluetooth connection for when I play Pandora on my Android through the car radio is that it's not quite as clear sounding as it is when I play the Nano through its USB port. Though that may be because my Bluetooth connection to the Sony is via an FM channel since my Sony is one generation too old to have its own built-in Bluetooth.