Link - On The Size of the Speaker Cavities in the iPad Pro, from Daring Fireball
It appears that the Pro’s self-balancing, four-speaker audio comes at the cost of battery capacity. Based on our measurements, the speaker enclosures occupy about half as much space as the battery. That’s space that could have potentially been used for an extra 50% battery capacity. We’re sure Apple was very careful setting the balance between battery capacity, weight, and sound quality.
A heavier iPad Pro with worse sound quality from its speakers would be a worse device, no matter how much longer the battery would last. Lots of the trade-offs are very tricky — like how thick to make the overall device. This one was not a tricky trade-off.
I haven't used the speakers in my iPad Pro, other than playing along to Taylor Swift with the piano in GarageBand, but thats because I still use my Mac as my TV. It's easy for me to say that I wish Apple put a bigger battery in the iPad Pro, but realistically I think Apple struck a good balance.