Your letter was more or less clean, without many grammatical errors, but I took the liberty to alter some things for greater readability. Overall, remember the KISS rule— Keep It Short and Simple
Dear Sir or Madam,
I am a customer of your department store and I am writing to complain about a product of your store that I found unsatisfactory.
Last Friday I bought a CD player from your store. This morning I was listening to music, when suddenly the CD player stopped working. I tried some other discs to check if something was wrong with the disc. Unfortunately, the player still failed to start. It is evident that the CD player is faulty. As the time since the purchase does not exceed one week, I would like you to replace my CD player with a new one.
I enclose the model number of the CD player for you to check, as well as my telephone number. Your prompt reply would be appreciated.
Student or Learner