If the USB-port can be used to play mp3's from a memory stick it's an option to convert the cassettes to mp3 (audacity is a free program to do that) and copy those to a USB-stick.

However, there are some points that need attention:
- A common 8 GB USB-stick can contain some 2000 mp3's. The player that that USB port is connected to should have an option to select albums or artists or tracks or whatever.
- You must use the volume control of the player. A USB-stick is just a storage device.
So that could be a bad solution.

And how about a smartphone with a headphone out? There are many players for a smartphone, and it's easy to copy mp3's to it.