Most of the time if your player is not working or making noise then it will be the 2 Drive belts.
Fits for SONY 300 400 CD Players
Note Sony Part number 421932601 is for the Sony CD Player's DOOR ONLY. Sony never used part number 421932601 for a Drive Belt as they are smaller.
Fit SONY CD Players Model: CDP-CX400 CDP-CX450, CDP-CX455 CDP-M333ES, CDP-M400CS, CDP-M555ES, CDP-CX777ES, CDP-CX985V, CDP-CX995V, CDP-CX300, CDP-CX333ES, CDP-CX335, CDP-CX350, CDP-CX355, CDP-CX555ES, and CDP-CX691
ALL Sony 300 CD Players have a total of three belts 2 Drive belts and 1 Door belt.