Running a sox spectrogram on a bandpass filtered wav file
sox is doing exactly what you tell it to do – it plots a spectrogram of the bandwidth you can represent with your sampling rate. The fact that most of your picture shows no energy just shows your filter is working, and that's exactly what one would want to see!

However, as you can see, your signal is solidly oversampled: You can downsample it to a much lower sampling rate after filtering! (and downsampling by a factor $N$ here really is just throwing away $N-1$ samples, keeping one, throwing away $N-1$ samples..., since your filter is an excellent anti-aliasing filter.)