Sox, mixing mono inputs into mono output (efficiently)
As far as I can understand, you want to add two signals, together?
I guess that you are missing -m switch.

Generate two test files (sweep and sine):

sox -Dr 16000 -c 1 -b 16 -n sine.wav synth 5 sin 1000
sox -Dr 16000 -c 1 -b 16 -n sweep.wav synth 10 sine 100:8000


Merge them together:

sox -m sweep.wav sine.wav -Dr 16000 out.wav


Using soxi out.wav you can see that produced file is mono:

Input File     : 'out.wav'
Channels       : 1
Sample Rate    : 16000
Precision      : 16-bit
Duration       : 00:00:10.00 = 160000 samples ~ 750 CDDA sectors
File Size      : 320k
Bit Rate       : 256k
Sample Encoding: 16-bit Signed Integer PCM


Now you can observe the spectrograms:

sox sine.wav -n spectrogram -o sine.png
sox sweep.wav -n spectrogram -o sweep.png
sox out.wav -n spectrogram -o out.png


Sine:



Sweep:



Merged together: