Is it possible for my iPhone to be hacked if it's not jailbroken?

It's not jailbroken, and I don't let strangers on my phone so no spyware apps were downloaded. Still, is it possible? Sometimes I hear what sounds like a whole crowd of people talking without any apps being open.

Uh
Just use your phone normally, don't go doing anything like that.

Possible? Yes
Probable? Not very

There's a company called Cellebrite that makes devices that are used to 'crack' passwords of the most popular smartphones and/or to extract their data. So yes, your phone can be hacked. After all, pretty much any devices can be hacked. Apple is generally pretty good when it comes to security, so it is unlikely (but not impossible), that your phone will be hacked. Just keep in mind that a lot of the "hacks" that occur come down to social engineering.