Finding audio data in recording
Knowing the range of frequencies is a good start and yes a bandwidth filter will certainly help you isolate them but you must consider the overall level of background noise vs signal noise.

Performing an FFT on filtered (bandwidth or otherwise) data will only yield results on the frequencies that are left over! This will either be of use to you or not.

If the number of frequencies that you are after are not that numerous, a collection of IIR (infinite impulse response) filters might be a better choice. 

Consider DTMF tones used by telephones and cellphones, typically IIR filters have found their way in software based detection of these dual-tone based signals. There are online resources for calculating each of the  filter's co-efficients. You must use one filter per frequency that you are after.

Here is an iPhone version of source code for DTMF decoding using IIR filters

The file DTMFDecoder.m contains the bulk of the code with the co-efficients defined near the top.

Without knowing any further detail, this is all I am able to help you with.