Frequency change on audio

[FKrlr.2155]

#define F_SAMPLE 48000.0f
#define F_OUT 4000.0f
#define PI 3.14159f
 
for(uint16_t i = 0; i < sample_N; i++)
	{
		sin = sinf(i * 2 * PI * sample_dt);
		
		dataI2S[i*2] = sin * 5000; 
		dataI2S[i*2 + 1] =sin * 5000; 
 
	}
	
	HAL_I2S_Transmit_DMA(&hi2s3, (uint16_t *)dataI2S, sample_N*2);

#define F_SAMPLE 48000.0f
#define F_OUT 4000.0f
#define PI 3.14159f
 
int a = 1;
 
for(uint16_t i = 0; i < sample_N; i++)
	{
		sin = sinf(i * 2 * PI * sample_dt);
		
 if(a%3) {
		 dataI2S[i*2] = sin * 5000; 
		 dataI2S[i*2 + 1] =sin * 5000; 
 } else if (a%2) {
 dataI2S[i*2] = sin * 3000; 
		 dataI2S[i*2 + 1] =sin * 3000; 
 } else if (a%1) {
 dataI2S[i*2] = sin * 7000; 
		 dataI2S[i*2 + 1] =sin * 7000; 
 }
  a++;
	}
	
	HAL_I2S_Transmit_DMA(&hi2s3, (uint16_t *)dataI2S, sample_N*2);

I am using the very above code to send sin wave to audio output and it perfectly creates sound. But I want to randomize the vawe and what I did is above. But it just generates the same sound. It does not produce sin*7000 or sin*3000. If I say only produce sin*3000 it does that. What could go wrong?

[Tesla DeLorean]

You are changing the amplitude, not the frequency.

[FKrlr.2155]

As I understand,

HAL_I2S_Transmit_DMA(&hi2s3, (uint16_t *)dataI2S, sample_N3*2);

I need to change the sample_N here to change frequency but can I do that dynamically during audio generation?

[Piranha]

To generate a single period of a sine with different resolutions, which at a constant sample rate translates as different frequencies, probably you want this:

sin = sinf(2.0f * PI * (float)i / (float)sample_N);

[FKrlr.2155]

Genereted sound is still uniform and do not change its frequency playing. Sound only changes when I change

HAL_I2S_Transmit_DMA(&hi2s3, (uint16_t *)dataI2S, sample_N3*2);

sample_N3*2, here.