A failing battery can have enough to operate low demand items like lights and audio but not have enough left for the starter.

I see. Is there any way to me to test and know for sure if it's battery or starter?

When my battery was 14 years old it started the car as usual, but speedometer and rev counter showed impossible readings varying between 0 and 90 mph, O and 6000 rpm.

I drove it to a trusted battery supplier, the tested the battery, declared it to be failing and fitted a new one while I waited.

A dud starter would not crank for any length of time.