Sample rate changing audio pitch
A couple of things jump out when looking at your diagram:


The first low-pass filter has a decimation of 4, so the sampling rate at its output is 256 kHz. However, you have set the sampling rate of the FM receiver and the second LPF to 224 kHz. These should be 256 kHz.
The cut-off frequency of the first LPF is 150 kHz. This means a total passband of 300 kHz. This is larger than necessary, so you'll end up picking up more noise than the optimum.
You multiply the audio output by a constant factor of 2. This may be too little, or too much. I'd suggest using a slider to control the volume. If you connect a scope sink to the audio sink input, you'll be able to easily get the optimum gain: you want the signal to be withing [1,-1].