Hello Gabriel,

1) Vacation is a great time for (or to) people have some rest from their daily routine;  You're right: for.

2) If I win a good amount of money, I'll buy a lot of cheap electronic devices or cheap electronics.   (?); Electronic is an adjective, so with a noun it's okay.

3) The old lady called me for (or to ) an interview.   (?); You're right: for.

4) Music relax (or relaxes)  the mind and shake (or shakes) the body.  (?)    Music should take the third-person singular. relaxes and shakes.

5) Is there any mistake in the following ?

- Music helps me to relax in a difficult moment of my life.; Not wrong, but if you use simple present, then there is more than one moment, so I would say "in difficult moments in my life."

6) Should we say I hate hard (or heavy music)?    hard rock or heavy metal music