So far there hasn't been a resolution that I'm aware of for keeping prim media sound confined to the source parcel but in preferences, under sound and media, you can set the 'Media source volume rolloff distance' so that you won't hear sources beyond that distance. It allows you to specify a distance at which the source starts getting quiter and a distance at which the sound disappears.
Evidently, this option is only present in the Firestorm viewer preferences now but the rolloff distance can be set in viewer 2 under the advanced tab, debug settings, audiolevelrolloff.