That is NOT for wake up, just doing a test of disaster prevention system
as announcement of time/time signal(calling 時報/jihou) for 6:00 o'clock.
Yakushima is surrounded by sea already you know,
so happend ferry/flight operating canceled by bad sea and wind condition a lot.
also rainy and tyhoon season will be bad weather a lot everyday tests is required.
Other times also using for testing everyday as announcement of time.
I heard from some friends who living in another region's sea side large town,
public speaker tests are doing only once on every weekend noon like this.https://youtu.be/qwxegL5VjaM
(for time signal by chyme or music and not both types)
Basically, inevening public annoucement with music a lot of small towns long years.
We(most Japanese)'re thinking that is one of
"sign/evidence of country side/rural area" as common sense of Japan mostly.
(but some urban cities also playing every evening even in mid Tokyo)
And in small/rural town's pubic important announcements
by town speakers is the fastest usual, very convenient as infomation spreading system.
However, sometimes "not smart" for temporary visitors/travelers we know :)
Yakushima some years ago was playing every night at 21:00,too. (now cannceled)