Formal:
* J'aime votre voix. (This is the most literal and formal translation.)
Informal:
* J'aime bien ta voix. (This is a more casual way to say it, using the informal "tu" pronoun.)
* Ta voix est sympa. (This means "Your voice is nice.")
* J'adore ta voix. (This means "I love your voice" and is quite strong.)
More poetic:
* Votre voix est magnifique. (This means "Your voice is magnificent.")
* J'aime le timbre de votre voix. (This means "I like the timbre of your voice.")
The best way to say it depends on your relationship with the person and the situation.