This comes with a strong spoiler alert up front. Only watch this, or read further, if you have not seen the conclusion to Halo 3: ODST, or don’t mind seeing it revealed.
A profile by the Seattle Post-Intelligencer, calling Marty O’Donnell the John Williams of the Halo series, turns up this tidbit: He wrote the Flintstones Vitamins jingle (which you now have stuck in your head) and his kids sang the chorus.