Country, I'm really sorry you are going through this with your daughter. My son (now 7) sounds similar so hopefully this post will help.
My son didn't speak/babble or say anything intelligible at all and we realised that he had issues when he was around 15 months old. Initially we were fobbed off by our not so wonderful GP with lots of talk about lazy boys, so I took on a private SLT who helped him to make some initially sounds. He has low muscle tone in his mouth, tongue, trunk and fingers and his low oral tone made forming sounds very difficult. We worked on some exercises which helped (pulling faces, sticking out the tongue, moving it up down and around) and it gradually helped.
He had his hearing tested which showed significant glue ear. We saw a harley street ENT who recommended grommets, but despite raising the issues of mouth breathing, told us that adenoids are no longer removed. There is family history of enlarged adenoids, but they refused to look at this, so they just monitored him, and eventually gave him grommets when he was 2.5. Nothing much changed despite many promises that he would now start to talk.
We then moved overseas, and have been able to do a lot to help. He's had his adenoids out and new grommets which made a HUGE difference to his clarity of speech and he actually acted as though it was the first time he'd properly heard the world around him. Please, please push to get this looked at - it's a quick, simple surgery and the results are almost instant. Do not be fobbed off - they can do a quick head x-ray to check the size of the adenoids but if she does have issues with mouth breathing, and speech, it's not a hard leap to make that she also has enlarged adenoids affecting her hearing and ability to breath normally.
We also saw a cranial osteopath who worked directly inside his mouth to help him increase awareness, mobility and sensitivity in his mouth and particularly his tongue. Again, very, very helpful to his clarity and diction.
Also perhaps look at diet. Does she often have a snotty nose? Sometimes children with speech issues also have food intolerances, pariticularly dairy and gluten. We've had big success with speech since removing both.
A good fish oil is essential - have a look at the Nordic Naturals pro range - we give 2 EFA and 1 EPA daily and have for the last 5 years. We see definite benefits with clarity (notice when we miss a few doses).
Apologies for this essay, but I hope it helps a bit. I've been there, I know how hard it is. Keep pushing for the help you are entitled too. It's very hard to have a child who can't communicate and at 7, my son is still not 100% but he's come on a long way. Happy to pm if you want any more specific info.