This page of the Spider-Man Miles Morales guide contains a walkthrough description of a short side mission Back to the Beginning, in which you will get a Purple Reign Suit. This mission will be unlocked after collecting all sound samples.
Rewards: 100 XP, Purple Reign Suit
After collecting all the sound samples, you will get a notification about the gift that Prowler prepared for you. The mission will not be available from the FNSM app - check the map to see where to go to start the quest.
When you get there, press R3 to scan the environment. Go through the grate to Prowler's secret hideout. You can browse through his stuff for a while. Check what items you can interact with, then go to the cabinet and read the message left by your uncle. You will receive a new Purple Reign Suit.