These are the 4-digit numbers that you normally acquire by defeating Imperial Walkers (gold floating robots) on the Goldoba. Thanks to WiredKnight
Considering how companies are now adays, it's hardly even a surprise that Namco's infamous Pac-Man and the likes would appear in Baten Kaitos. Oddly enough, however, the cameo appearances take more work than one would think...
In Reverence, Picture Book Town, head to the house opposite the fortune teller's house, and examine the two drawers. You'll obtain the Monolith Pen and tri-Crescendo Pen. These refer to the designers of the game, Monolith Software and Tri-Crescendo.
Use in the order I list.
To unlock the last 6 songs in sound test mode, let the "Shampoo" Magnus age for 336 hours (2 weeks) game time. It takes a very long time, but it's the only way to unlock the last songs in the Sound Test.
The Magnus "Rare Action Figure" is really KOS-MOS, the android from Xenosaga. The Magnus can be gotten by exchanging your 42nd Constellation Magnus in.
A boss in the name is called "Gnosis". This is an obvious reference to the main enemies of Xenosaga, the Gnosis.