What two vowels when placed together will make one sound?

The usual vowel pairs are

AI - makes AY or AIR sounds

AU - makes AW sound (equivalent to OR in British English)

EA - almost always long E, but often AIR with an R after it

EI - almost always long E

EU - often a long U (long OO)

IE - almost always long E or long I

OA - often long A

OE - often long O

OI - the special OY sound

OU - long U or OW sound

OW - the special AU/OW sound

OY - the special OI/OY sound (blend of O-I-E)