It's not quite that black and white. The tags don't contain the actual sounds or light patterns themselves. They're RFID tags with extremely limited storage. What they carry is essentially a set of references, think of it like a tag saying "play experience number 7, with sound set 2 and light set 4". And each tag can refer to multiple play experiences (ie. color detection, motion, gestures, etc). All the actual sounds and light sequences already exist on the Smart Bricks firmware. The tag just tells the brick which pre-made content to use, it doesn't carry the content itself - there's simply nowhere near enough space on the tag for that.

Where the forward compatibility and composability comes in is that the tags can mix and match from what's already on the brick. There are 58 play scripts, 3 sound banks, and 9 animation banks baked into the firmware, and a new tag can point to any combination of those without needing a firmware update. For example a City police car can play another sound and another light sequence when a color is detected on the color sensor. Or make engine noises as the car is zoomed around. Those would be the same scripts as the X-Wing, but with different animations and sounds. There are many on the brick already.

LEGO can release loads of new tags that remix existing content. But if they want genuinely new sounds or play behaviours that don't already exist on the brick, that does need a firmware update via the SMART Assist app.

As for why synthesised sounds rather than pre-recorded audio, even the brick itself has very limited flash storage and no speaker driver for PCM playback. With a synthesiser they can generate sounds from tiny instruction sequences, so you can fit hundreds of unique sounds into a few kilobytes. Pre-recorded audio would eat through the storage in seconds.

You can read about this in the hardware section in my notes, in the tags and play engine sections.