There are two possible pronunciations for Amontillado, for which different people will make a case. I will present the arguments and you may decide which is best for you.
1 – Amontillado, where the ll is prounced “l” as in ballad. This pronunciation follows the Italian. The case for this pronunciation is that the story takes place in Italy and the main characters are Italians, thus they would use the Italian.
2 - Amontillado, where the ll is pronounced like a y as in yah! This pronunciation follows the Spanish “ll”. The case for this pronunciation is that the wine is Spanish, originates in Spain and would therefore have a Spanish pronunciation.
In either case the vowels are pronounced the same.
The question of using the Spanish or the Italian is, of course left to the reader and you may choose whichever justification suits you the best!