I don’t have an amplifier yet so I was wondering if a radio could work as an amplifier until I get one?
That might, but a better alternative that can play tones & messages on any of your speakers & speaker/strobes far more easily is this 20-channel MP3 board (you’ll need an SD card for it however):
20 Channel MP3 Sound Board with 2x15W Amplifier | Electronics123
If this is for fire alarm speakers you will probably need to bypass the transformer. Most fire alarm speakers operate on 25 or 70 volt circuits. Stereo output is quite a bit less.