You can adjust the sound from ‘settings’. I think the ignition has to be on or the engine running to see this option. There were only two loudness options when I last looked. I can not remember exactly where it is but you will have to be a sado like me and go through all of them in turn till you find it.
On the MR2 owners forum, a very similar thread came up. The MR2 Mk3 indicator “tick” is very quiet and one of the people on there fitted a buzzer something like that. Apparently it worked very well. The way it was wired up, it beeps in time with the normal tick sound and isn’t annoying (they said).
Another suggestion was just to wire another relay in place that doesn’t actually power anything. So, all it does is open and close making a tick sound.
However, with the ND sound being volume adjustable, it’s probably mostly, if not entirely, synthesised and IMO it seems a bit of a shame to try and bolt on extra noisy bits that are not as well integrated into the car’s system.
From the manual above it shows an alarm/speaker behind the dash which is switched by a transistor controlled from the CAN. Not sure what can be done, thought maybe someone had changed the alarm/speaker for something louder. Naturally I don’t want to risk damage so wouldn’t do it unless some after market crowd has sorted it out
There’s stuff on the internet to hack the Mazda BCM which may make it possible but that’s way beyond me.
Edit: the Mk4 manual mentioned is on the MK4 wiring diagram thread.