These days most of the audio we want to hear is on our mobile devices. If you have an older boombox with a few parts and some time you can create Bluetooth connectivity as long as the boombox is designed for use with an infrared remote control.
Electronics enthusiast site PC486 shared how they reverse engineered the infrared remote signals for a Panasonic SC-AK33 boombox and put together an ATtiny25 microcontroller with an infrared LED diode and programmed it so that commands streamed from the author's phone via Bluetooth would be received by a Logitech Bluetooth wireless speaker adapter and resent via Infrared. This means that no modifications to the original boombox were necessary.
For full details on the project as well as photos and the code for the repeater visit the source link below.