I am running Bike Or Die! v. 1.5 on my Palm T|X handheld.
If I set "Audio off" in system Preferences and set "Obey sistem settings" in Sound options within Bike Or Die!, the portal "swoop" sound is still played. No other game sound is played.
thank you for reporting!
it seems like very old bug - i just tried v1.4 and it does the same (but obviously not for portal effect the bug also affects the freewheeling sound)