Power Blade

Sound Test
At the title screen, hold Left + A + B on Controller 2 and press Start on Controller 1. Press Up/Down to select the music and A to play it.

The sound test looks a bit different in the Japanese version, and most of the music tracks have names instead just "BGM01"/"BGM02"/etc.

Hidden Credits
Present at in all versions: TOSHIKAZU IWASA

Regional Differences
The US version is pretty much a completely different game compared to the Japanese one: different hero, different enemies, different levels, etc.

Stage Intro
In the Japanese version, this appears by remaining at the title screen. In the US version, it appears upon getting past the title screen. Alternately, if you let the game sit on the title screen, you get a new story intro.

Map Screen
The map screen was completely overhauled. It's now far more helpful, as well as more detailed.