How to improve the quality of the receive sound?

I am doing an experiment which uses laser to detect sounds. The struct include: Speaker connected to cellphone broadcast music or any mp 3 sounds. A very little mirror is pasted on the speaker. A red laser is emitted to the speaker (on the mirror). Then A receiver (photo diode）receive the reflected laser, then the signal is amplified , then to another speaker. The original sound is reproduced through this way. Now the distance is 50 meters. The question is, the sound is not clear, if it is a music, I can distinguish melodies easily, but I can not tell the lyric. So how to design some channel equalization, I want to use Matlab to processing the receive sound. (through Mic input ) (1) Using what signal to get the system parameters? ( I can put it on the cellphone) (2)In what conditional, the sound can be recovery? (3) If the distance is changed, or something unknown changed, (each time, I have to alignment the laser, so little change happens) is there any adaptive method to do so?