your degree of interference when rigging up external speakers to your 396 will depend alot on your 396's location and what antenna it is using.
not to mention the type of external speaker!
i have had no problem in the past just plugging a set of computer speakers into the audio-out jack of the 396 and listening to a local trunked systems or conventional frequencies.
HOWEVER
when you get to sensitive frequencies, like the Civilian Air Band, you might experience some problems. just remember that the speaker wire can also act as an antenna in some cases. also, depending on how close your scanner is to a computer, your computer may generate some RF that can interfere with Airband, 150Mhz, 400Mhz, etc.
i had such a challenge with linking up external speakers to my 396 (which was plugged into a huge yagi antenna) that i finally just dug up some old TV-to-TV 2.4Ghz wireless links and ran the 396 -> 2.4Ghz link -> 2.4Ghz link -> entertainment center with large receiver and speakers.
in my case, the antenna and frequencies i was scanning were so sensitive, anytime i WALKED NEARBY with a laptop, the scanner would lock on RF bleed.