Dear Bob,
I am writing to express my apology to you. Several days ago, I borrowed your music CD when I lived in your house. Unfortunately, after I came back from Canada, I found it in my luggage. I was in such a hurry that I forgot to return it to you .I will send it to you by post or express as soon as possible. If necessary, I will compensate for any troubles it may cause.
Once again, I feel so sorry for any inconvenience caused. Please accept my apologies.
Sincerely yours,
Li Ming