You need to use a low pass filter (taps) in you resampler. Look at the spectrum after the resampler and this will be obvious to you. A lowpass filter with a cutt off at 3-4kHz or so should do it.
This cannot be the problem, because rational_resampler automatically calculates a filter if taps are not specified:
(I also checked the GRC file, and the fractional bandwidth is also None (auto-set) despite appearing as "0" in the screenshot.)
As a next troubleshooting step I would recommend adding GUI sinks at various parts of the flow graph so that you can check the signal integrity. (For an example, consider adding after the float-to-short a short-to-float and QT GUI Sink — to ensure the signal hasn't gotten lost in quantization noise and just needs to be amplified before converting to short, as well as checking that the bandwidth/resampling is good.)