You're absolutely correct - so long as Sony engineers a different manner in which to perform the same function, namely, in-game personalized soundtracks, its not patent infringment (though, that wouldn't necessarily keep Microsoft from trying to prosecute Sony). Keep it tied to XMB and they're likely all good.
Essentially, the way it all works is in the claims of the patent (you will see these w/ numbers representing each claim). If Microsoft's patent claims its invention does ...
If Company A owns a patent, or patents, that, if licensed, would engender a respectable (or, simply respectable) sum(s) of money and that patent, or patents, was (were) being infringed upon, believe me, Company A will prosecute Company B, C, D, E, etc. -- this is likely especially true w/ a company like that of Microsoft; one which allocates in its annual budget an enormously large amount of money to the prosecution of said patents and the research performed by its legal team for the infringm...
I find it very likely that in all of Sony's patents related to XMB there has to be, IN THE VERY LEAST, one claim asserting in-entertainment personalized soundtrack capabilities, if not more -- this would include gaming. If so, no patent infringment (likely) exists (maybe other things w/ other companies, who knows in that regard). Sony's XMB patents likely date back a long time ago, citing many a many prior inventions regarding music capabilities. They'll be good.
By the way, ha...