Hello Vu Nhu Khanh (which is your first name?)
I would suggest you do the following:
download this e-book from the Analog Devices website, and study carefully particularly the first few chapters. It will teach you the fundamentals of sampled data systems. http://www.analog.com/processor...index.html
Next, most importantly you should DECIDE what audio bandwidth you WANT.
Multiply the maximum desired audio frequency by two, and use a timer and interrupts to achieve good, steady sampling at such a frequency and use a slightly higher baud rate than your sampling rate actually requires to send the data across.
Design a filter to cut off any frequencies above the maximum you have set.

Using jnb ti,$ and the likes such as delay loops are terrible techniques which will prevent you from ever writing a proper program. And this application does need one. USE INTERRUPTS, NOT JNB TI,$!

Good luck!