My cat is meowing when she's in the litter box - is anything wrong?

Hi there!

We have an indoor female cat, 7 years old, and sometimes, when she's in the litter box, she's meowing. I asked the vet and she said that as long as she's not spending too much time in there (but what is too much time for a cat??) and as long as she both pees and poops, then it should be okay. Her poop is always firm - I don't know if it can be too hard or what we should do to make it softer in that case? She drinks a lot of water, eats dry food and once per week she gets wet food.

Any inputs?

Edit: Now, one week later, we have changed the litter box to an open one and talked to the vet about it. I also managed to record it one time (pretty weird to film a cat pooping...). She thought we should give her 1 ml of Lactulose (I have no idea if you have that in the states or anywhere else, but basically it's a mild laxative also given to babies and such) every morning and evening. We haven't caught her meowing in the litter box, but we have been at the beach and such and therefore haven't been much home. The vet said that if it helped, it could be an idea to give her dry food specifically for sensitive stomachs otherwise we should bring her in for a check. So we'll see how the next couple of weeks will go.