Just get surround sound and play your own music by playing it through something else. I've been rewiring my own music into games since the original playstation came out. I usually use my old cheap surround sound and use a headphone jack and play pandora for certain games like racing games.
I think they only put custom soundtrack on the box when you can add your music from an in-game menu and not from the XMB. Like if you could make a mixtape to control like a radio station, then I believe that's what gets the "custom soundtrack" remark.
PSN: Sychosymatic Platinums: Rock Band 2, RE5, WWE LoW, Batman: AA, Uncharted 1 & 2, WWE SvR 10, Heavy Rain, MW2, AC2