Wow! That's quite an Idea. Hunting?
This project can quickly become complicated thinking at a micro level. Macro level thinking, the project is a combination of a few circuits.
There are two key components in this concept, and the entire concept will end up revolving around the decisions made for these two key components. The components are amplification and the transmitter used to control the audio. The princilples are also dependant upon your confidence and knowledge with building electronic circuits.
I look at the concept like this:
1. Audio platform --The audio player.
A. Is the flash memory played by a flash player?
B. Can you burn the audio to a CD? If so, you may be able to use a portable CD player as your platform.
2. Amplification--
A. How much amplification will you need, according to the built in dB level of the audio platform? I assume CD players play max volume around 100dB's? I could be incorrect. Based on that assumption, you'll only need about 20+dB amplification. There are tons of low cost ($10-$20) circuits that will fit the bill.
B. How much noise are you willing to accept? Probably not much, based upon the 16bit 44.1khz sound quality. You may need a filter circuit to clean up the audio after the amplification. This can be added later if you so chose.
C. The more amplification, the more battery usage. General assumption, you'll need a seperate 9V for each amplifier stage.
D. How much are you willing to spend on amplification? Not much, okay, you're going to have to live with some backround noise.
3. Transmitter and receiver package--
A. 200 - 300 yards is pretty far for a simple transmitter circuit. You may be getting into pretty complex and expensive circuits in order to effectively transmit for this range.
B. What kind of transmiting medium are you looking for? Infared? FM? AM? Ultrasonic? Laser? I'd have to say Infared and ultrsonic are out of the question, based upon the 300 yard range. Laser gets expensive, so AM and FM are the choices I suppose.
C. I'm not in tune with transmitters and receivers. I do know that you're looking at a fairly robust transmitter (due to the range), not some $10 transmitter. Also, I do know that you're looking at a fairly robust receiver because the receiver will be used to control the functions that you need, volume, track play, etc.