Kia Sedona won't start after engine warms driving it makes a whistle sound took 2 garages 1st said 2 split Piston a dry noise in sump 2nd gearbox problem and it cutting out was a sensor in the gears?

The whistle is most probably a loose vacuum line
that will make it hard to start when hot as the mixture will be out
pistons loose the top at a ring groove and you will know that by the banging noise
pistons seldom split in normal cars as there is a gudgeon pin to hold then together
any thing wrong with pistons and the engine will be very noisy and you will know it.
The whistle may be from the vacuum line to the transmission that changes gears at certain rpms
run the fault codes first the find an accredited shop that has experience in your make of vehicle.

The already posted replies are good advise but i would like to know what tests these garages are carrying out , so if there is a split piston then the compression for that cylinder will be down so i'd ask for the compression test readings , if this is the case it would have problems starting whether warm or cold , also ask if the OBD codes have been read and if so get the codes from them, will begin with a P, then some here can check them for you , hope this helps , dont rush into anything until you have the test readings not just the garage opinion (thats the synic in me , sorry once bitten twice shy ) so always get max info from any garage etc

Get another opinion from a 3rd shop. Might cost you a little, but that is cheaper than $$$ on an engine. If is really is an engine then get a price from a wrecker on a good engine and see who you can get to switch it out. If the total $$ is greater that 1/2 of the price of another complete Sedona of the same age do not repair, buy another vehicle. If its a tranny minor repair - do it. If major repair - same rules on the costs apply.

Main words there... split piston!?!? so you'll need a new engine or a costly rebuild... that right there will make or break you for another replacement vehicle

and transmission problem is probably a shift solenoid is bad or the wiring to it is broken... less expensive than the whole engine rebuild

either case, i would scrap it if it's older than 7 years