Well, as I mentioned, a big part of the game's content is online, and a big part of *that* content looks to be user-generated and is constantly being updated. (The 4 songs you picked out, and in fact all the songs I listed are only available online.) Just today, July 3rd, there were 38 songs added by users (granted, some of theme were the same song but with different arrangements). I'm using this site, and more specifically this page (along with Google Translate when needed) to keep up with the new stuff as it seems to be updated quite speedily. It also allows me to see which songs are duplicates so I can weigh one against the other.

The first Band Bros. game is probably the primary reason I was in love with the DS in its early life. It was a Japanese launch game that was *supposed* to come out in English as 'Jam With The Band', but it never happened.

Edit: There were actually 52 songs added 'today', which has ended in Japan already. My list of 38 was from this morning :P