stereo equipment effects the value of a car on a total loss work sheet...when cars are valued, all options are taken into consideration...in your case the option for CUSTOM A/M AUDIO SYSTEM would be used, this adds a point rating to the final value, not much, but in most cases if there is no H/U and Speakers it will change the value a couple hundred dollars depending on the age of the vehicle...
if the insurance company has already seen the car, and written a value on it based on what is inside the car, no you CANNOT remove the items...
if they have yet to see it, then yes, you can take out whatever you like, just keep in mind it may make the difference between an extra $300-500 based on the age...if its newer (2000+), its a big deal, if its older, it matters very little...
in most cases we'll let people take the subs and amps and just mark it down as a mid grade custom a/m setup...
although keep in mind some companies have a "bolted down" law, if it's bolted down, it's theirs...