I’m a Speech Pathologist and agree with the suggestions that others have posted. Positive reinforcement for when she is using her ‘inside voice’ is really important. Star charts, a special treat, a cuddle from mum or dad, access to a favourite/special game/toy etc.
First though you need to make sure she knows what you mean by an ‘inside voice’ Make a game of it and talk or shout yourself really loudly explaining that this is your outside voice, but when inside you use your ‘inside’ voice. Demonstrate your inside voice. Get other members of the family to do this also. You can also experiment with musical instruments, the tv, stereo etc making it loud and soft so she understands the concept. It is really important that she does learn how to use an inside voice, as by yelling, she could potentially be damaging her vocal folds which in the long term can lead to things such as nodules (like blisters on the vocal folds).
I wouldn’t discount her hearing though. At this age, children’s hearing does fluctuate with things such as ‘glue ear’ This can make it difficult to hear the volume of your own voice so she’s not getting any feedback about how loud she actually is. Have you noticed her turning up the tv louder, sitting closer to the tv or asking ‘what’ or for repetition a lot?
Remember lots of praise though! And random praise. When you notice her using her inside voice tell her. So often parents pick up on what their child doesn’t do but don’t seem to notice when they are doing what they are meant to be doing.