Leave the game going and let the animated intro play through once. On the second time the animation plays, you will hear an alternate song.
Blue Spheres
Once you get all the medals from the Bonus Stages, you'll unlock Blue Spheres in the Extras menu, where you can play all 51 Blue Sphere Stages.
Debug Mode In Sonic Mania
By gathering all seven Chaos emeralds and Beating the game, You will unlock debug mode. You cant only use it by pressing Triangle on the no save mode of the game, then turning it on, afterwards, in game pressing triangle will Toggle it on and off and O will change the selected Item.
Fight the true Final Boss
As Sonic, collect all seven Chaos Emeralds before finishing Titanic Monarch Act two. After beating the normal boss of Titanic Monarch Act two, Sonic will be teleported to the true final boss.
Level Select
Depending on the gaming platform used, entering level select is different. Switch allows it from the title screen, others from the save screen. PS4 does the latter. To navigate the level select screen, use the d-pad. To switch characters, press Square. To add Tails to Sonic, press Triangle while Sonic is shown next to the Sound Test.
Medals
Collect medals by beating the various Blue Spheres special stages throughout the game to unlock the following rewards.
Medal
How to unlock
Blue Spheres
32 medals
D.A. Garden
26 medals
Debug Mode
16 medals
Insta-Shield
6 medals
Knuckles Mode
11 medals
Mean Bean
21 medals
Super Peel Out
1 medal
Sound Test Codes
On the Level Select, play the following music tracks on the Sound Test to enable special features. A confirmation sound will be heard when the correct order is played.
Code
Effect
01, 09, 07, 09, 00, 08, 01, 01
Changes all small animals to squirrels.
01, 09, 08, 09, 00, 05, 00, 01
Effect unknown.
01, 09, 09, 02, 01, 01, 02, 04
Infinite continues.
02, 00, 01, 07, 00, 08, 01, 05
Enables flight when in Super form (double-tap jump)
02, 00, 01, 08, 00, 06, 02, 03
Start game in Encore Mode (DLC required)
04, 01, 02, 06
Start with all Chaos Emeralds.
06, 02, 01, 04
Disables Super Sonic theme when transformed.
09, 00, 00, 01
Enables all special moves for Sonic (Drop Dash, Insta-Shield, Peel-Out)
Videogamedunkey Mode
In Hydrocity Zone, grab onto one of the hooks, and input the Level Select code from Sonic Three & Knuckles (Left, Left, Left, Right, Right, Right, Up, Up, Up). You'll hear a ring chime, and when you pass through a bumper with a white glove, the sound effect will change to that of Videogamedunkey making a noise.
Alternate Song in Opening Anim...