it's mostly trial and error, and personal opinion
Some handsets I've felt adequately loud for everyday use my parents felt were too 'soft' or 'muffled'. I've used the really basic Nokia 2610 (released by ATT) and it seemed like a phone which was (by price) proportionally good in that regard. For non-smartphones I've also liked the Motorola Vx (RAZR variants) for good sound quality, but again, YMMV. My ears are different than yours. Also, the network you are using a particular handset on can make a bit of a difference. Even service from the same carrier can sound a little better or a little worse depending on where exactly you use the handset. In this case, you may not like the way your calls sounds on a RAZR on ATT but like them on Verizon, etc. In this sense you could benefit from stating on which network you plan on using the phone you had in mind.