I've just done the update - this could be a placebo effect buttttttttt - It sounds like they've boosted the treble quite a bit - talk to Siri and he'll speak with less bass.
Now, from my experience - when the treble is increased it makes the volume sound louder, however, this is just an effect because the pitch is higher. - the volume is actually still the same.
I honestly think that the speakers are the same volume, however, they do sound a bit better but not as good (or as loud) as my previous 2019 13 inch MacBook Pro.
I was thinking that maybe Apple put smaller (cheaper) speakers into the M1 to save costs (because of all the £££ they spent on R&D for the new chip) - they could easily make the M1 speakers the same volume as intel MacBooks (through a software update) but the speakers would get damaged due to the extra power (chemical power(battery)>electrical potential energy) driving them.
Let me know if I've got anything technical wrong here, GCSE science is as good as it gets 😅:).