Mundane series sounds pleasant (6)
I believe the answer is:
dulcet
'pleasant' is the definition.
(similar in meaning)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dulcet that I've seen before include "Sounding sweet, mellifluous" , "Sweet, gentle (tones)" , "Mellifluous, pleasing to the ear" , "(Of sound) sweet and soothing" , "Sweet - melodious" .)