Here is a list of games I tested with my brand new RetroDuo:

NES (US)

1943: Works (but sound issues)
Astyanax: Works
Batman: Works
Bayou Billy: Works
Bionic Commando: Works
Captain Skyhawk: Works
Castlevania 2: Works (but sound issues)
Commando: Works
Double Dragon: Works (but sound issues)
Double Dragon 3: Works (but sound issues)
Dr Mario: Works (with very slight sound unbalance)
Dragon Spirit: Works
Dragon Warrior: Works
Fast Break: Works
Gauntlet: Works
Gold Medal Challenge 92: Works
Golgo 13: Works
Kirby's Adventure: Works
Kung Fu: Works
Metroid: Works (but sound issues)
Mickey Mousecapade: Works
Ninja Gaiden: Works (but sound issues)
Ninja Gaiden 2: Works (but sound issues)
Pro Wrestling: Works
Rad Racer: Works
Rad Racer 2: Works
RC Pro-Am: Works
Shinobi (Tengen): Works
Spy Hunter: Works
Super C: Works (but sound issues)
Super Mario Bros 2: Works (but sound issues)
Super Mario Bros 3: Works (but sound issues)
Tag Team Wrestling: Works
Tecmo World Wrestling: Works
Tetris: Works
Tetris 2: Works
TMNT: Works (but sound issues)
TMNT 2 - Arcade Game: Works (but sound issues)
TMNT 3 - Manhattan Project : Works (but sound issues)
Track & Field 2: Works
Vice: Project Doom: Works
Willow: Works
Wizards & Warriors: Works (with minor sound issues)
Wizards & Warriors 2 - IronSword: Works (with minor sound issues)
World Class Track Meet: Works

SNES (US)

Super Tennis: Works
Super Street Fighter 2: Works
Capcom's Soccer Shoot Out: Works
Nigel Mansel: Works
Super Castlevania 4: Works
Street Fighter 2: Works
Axelay: Works
Pilot Wings: Works
F-Zero: Works
Drakkhen: Works
NBA Showdown: Works
Super Mario World: Works

Conclusion:

It appears that the NES side of the RetroDuo has a problem with the reproduction of sound. Indeed, the bass are much louder than the trebble and some audio is not played at all (i.e. Ninja Gaiden, Castlevania 2 ...).
This is very sad!

I hope that, somehow, there will be a fix of some kind...

Cheers