Pretty much anything you put in place of those OEM speakers is better than what's there right now. I'm serious. You can go to Walmart and get something for $20 off the shelf and it will be better.
Joel Sell (aka Million Mile Sprinter) posted a 1-minute vid to Instagram last week showing how easy it is to swap them out. I don't know how to access his short vids except via IG.
Thus far, we have taken three steps to improve our sound situation:
1/ Dash speaker swap-out as described above.
2/ Added a Pioneer head unit that kinda sorta has Apple Car Play. I will warn you that the ergonomics of this contraption are THE absolute worst I've ever seen. But with only the single DIN spot in the older Sprinters, there isn't much choice short of wholesale dash re-fabrication (which some owners have actually done but we decided against it).
3/ My husband installed two additional mid-range-y speakers in the center-bottom of the dash structure. These improved the sound, but not radically. Plus, if our dog happens to be snoozing between the cab seats, we have to be careful not to blast her head off with the speakers being so close. In fact, these speakers were part of my inspiration for
building a proper dog platform for the passenger seat, because I like to crank tunes and podcasts while on the road. So, I needed to get the poor dog off the floor.
The system is still woefully bad, however. It needs a subwoofer (dog going "Woof! Woof!" does not count). We might eventually get around to installing one in the headliner, because in our rig, there really isn't any other location we could use (some people install behind the driver's seat, but our wet bath is right up against it).
Edit: We also added some noise attenuation measures to the cab (description
here). T1N Sprinters tend to be so loud that even the best sound system would be undermined by road noise.