Hi,
So you have the STM working with 700kHz PWM frequency and software generated 7kHz sinewave.
Now you want to apply an external 7Khz.
What is your input sampling rate?
How often are your reading the AtoD output of your input signal?
Tom... 
Before applying external signal I am looking for line which will turn off modulation from 7 kHz generator inside of stm32.
Second two questions not sure, sample rate 100, and
"Dec 18, 2015 - 12-bit successive approximation ADC. Maximum ADC conversion rate is 1MHz and more than 2MHz in some STM32 families. A/D conversion range: 0 – 3.6V DC. ADC power supply operating range: 2.4V – 3.6V DC. ADC input range: (V_Ref- and V_Ref+ pins are available only in some devices and ."
This is the line which is connecting generator to modulator, what it is saying - no idea.
Now I need create the line which will connect PA6 to modulator.
dma_cfg.tube_flags = DMA_CFG_SRC_INC | DMA_CFG_CIRC | DMA_CFG_CMPLT_IE; //generator ON_OFF
S1 = old code line
S2 = new line to be created
And I think I stuck here.
Almost there, 1 error to fix.
error: return-statement with no value, in function returning 'int' [-fpermissive]
// open the input waveform file
in_fid = fopen( "input.pcm", "rb" );
if ( in_fid == 0 ) {
printf("couldn't open input.pcm");
return;
}