What's the word for words that roll off the tongue?

Solution 1:

"Phonaesthetics" describes the study of such things and the appropriate word would be "euphony" or "euphonious":

A pronunciation of letters and syllables which is pleasing to the ear. (wiktionary)

Edit in response:

The phrase "articulatory phonetics" describes "how humans produce speech sounds via the interaction of different physiological structures." Phonaesthetics describes the more abstract aesthetics associated with speech. "Phonetics" includes the physical motion of speaking. Therefore, I claim that phonaesthetics includes the physical aesthetics of speech. "Euphony" refers to the pure sound aesthetics but either of these two terms should work:

• phonaesthetics
• articulatory phonaesthetics

Solution 2:

Mellifluous, maybe?

I'm not sure if you are looking for an adjective that describes them, or a noun.