The base stereo in these newer Mustangs are barely good enough for AM news/talk radio. The base speakers and head unit are a testament to an engineers mandate to produce something by the thousands that does function as advertised, will last through the warranty period, and that's ALL.
There is absolutely NOTHING about these components that suggests 'high fidelity'.
You can replace the craptastic speakers with something decent, JL Audio, Infinit, Polk, Pioneer, Alpine, etc as a starting point. Only problem with that, is you then quickly discover the simply *awful* amplifier in the head unit. I call it "the little amplifier that couldn't". Turn it up, and you quickly discover the 'bass roll-off' where the amount of power you get from the rear speakers practically disappears. Why? Because the amp in the head unit has so little power, the engineers designed it to wring out every last watt, and it soon falls on its face. This 'feature' really is exposed when you try and compensate for the weak stock amp, by installing your own amp.
And now you finally realize 'the horror' of the sound quality coming out of the head unit. IT SUCKS!! Not sure which frequency you get to adjust for Bass/Treble, but even with top quality speakers and a powerful amp, you will get disappointed on certain source material when the bass goes boomy/muddy. No amount of fiddling with the tone controls will help, you just have to turn down the volume.
As the base stereo models do not have the 8" bass woofer in the bottom of the door, you have 2 choices. Install a subwoofer in the trunk, or replace your base door panels with Premium panels, and add mid-bass/bass woofers in the doors to compliment the speakers up top. The door panel route is pretty expensive, but I chose to take that route and not cut into the door panels in my '13 Boss.
I have incrementally upgraded the stereo by first replacing all the stock speakers with 6x8 JL Audio co-axials, then an old 4x75 amp which was failing, and now a new 4x175 Crunch amp. Sure it's way better now, but I am now saving up for an iDatalink double din dash kit, and head unit. http://maestro.idatalink.com/maestro...g-dash_en.html
How far you are willing to go (spend & modify) is entirely dependent upon what you listen to, how loud you want your system to go, and what level of clarity and precision you desire. If you've had the opportunity to audition any of the high end OEM systems you can get in an Acura/Lexus/Infiniti/Benz/Audi, you can see what kind of quality is possible when it's designed into the vehicle from the factory.
There have been plenty of posts/threads on how others have upgraded their Mustang car audio, and you can get some great ideas. One thing I came up with was to install my amp underneath my rear deck, suspended by tie wraps. It allows for easy removal if I want to go to the track or just as important, access convenience for tweaking the amp settings for crossover freq & input sensitivity.