Get a sound system, record the exhaust of your mustang, and play it on your sound system.
But realistically, I haven't seen many 4 cyl.s that sound good IMO. My friends '89 corolla had a couple holes in the muffler so he replaced it with the quietest dynomax muffler. It was quiet (but still a bit louder than stock), had a nice deep tone to it (for a four-banger that only had 3 cylinders working), and was relatively cheap. You could easily just grab a deeper-sounding muffler and get it welded in for pretty cheap.
yeah i already got a kenwood deck 6x9 pioneer 3 ways 6 1/2 2 ways and 12" dual subs and a kenwood amp for it lol yeah my dad said they had a magnaflow muffler in stock the other say he saw it at ford is that a good one ???