Frequency modulation of audio signals. Does increasing the pulse width of frequency tone result in larger operating distance?
It is very hard to give a single, correct answer because there are so many variables involved. Some things to keep in mind:


The main variable that determines the number of errors in the receiver is the ratio of energy per digit and the noise (this is the signal to noise ratio, or SNR). When you decrease the pulse duration, you reduce its energy and in consequence reduce the SNR. Increasing the pulse duration will increase the SNR and will allow you to increase the distance, all else equal.
Received power decreases with the square of the distance; for relatively large distances, this will dominate the SNR.
The technique you are using is called frequency shift keying (FSK). It is easy to implement but requires large bandwidth. Your audio setup has limited bandwidth (20 kHz in the most optimistic case). Reducing the pulse duration increases the bandwidth; at some point, your speaker/microphone combination will not have the bandwidth to properly reproduce and detect your signal.
Bandwidth considerations also suggest that you should shift your frequencies to around 10 kHz: at 17 kHz, you will soon hit the system's limit of 20 kHz. Operating at 10 kHz, you have 10 kHz of bandwidth to either side of your carrier.