Now in this scenario, although you are the customer, try to be polite first, 先礼後兵.  Vinegar won't help you to catch flies.  Never to threaten directy.  This will do you no good.  Used veiled ones instead.  They will get the point.

A Letter of Complaint

Dear Director of the Sales Department,

Dear Sir,

I am My name is Milton.  I am , an a regular customer of to your shop. I shopped often year because of the high quality merchandise and the reasonable prices you have.  However lately, I was quite disappointed.

Take for an example.  I bought a CD player yesterday from your shop.  However when I played my CDs, there was nothing but a very annoying noise blaring out.  I am now writing to you so that I may be reimbursed with a non defective one.

I sincerely hope that this is not too much of an incovenience for you to do so and have this matter settled amicably.  I will not stop until this matter is resolved.

Thank your for your prompt action.

Your regular customer.

 I venture to write you a letter about the broken CD player I bought from your shop yesterday.
The main problem is the poor quality of the player. When I use it to play all of my CDs, all my hear is loudness which totally drives me nuts. Honestly speaking, I am not making fuss over this matter, and there is much room to improve this situation. I hope you can provide me a new CD player or refund my money, or I will complain to the Consumer Association.
Milton

A Letter of Complaint

Dear Director of the Sale Department,
I am Milton, an regular customer of your shop. I writing to inform you of a venture to write you a letter about the broken CD player I bought from your shop yesterday.
The main problem is the poor quality of the player. When I use it to play all of my CDs, all my that I  hear is loudness which totally drives me nuts. Honestly speaking, I am not making fuss over this matter, and there is much room to improve this situation. I hope you can provide me a new CD player or refund my money.  Otherwise, or I will be forced complain to the Consumer Association.
Milton

[Your English is good.  Letters like this have a very specific language associated with them.  You actually did very well.]