Audio and Generated Video Keeps Desyncing Despite Audio Being 48000khz - c++

Question

So I'm writing a C++ program that will take a wav file, generate a visualization, and export the video out alongside the audio using ffmpeg (pipe). I've been able to get output out to ffmpeg just fine and a video with the visualization and audio are created by ffmpeg.
The problem is the video and audio are desyncing. The video is just too fast and ends before the song is completed (the video file is the correct length; the waveform just flatlines and ends, indicating that ffmpeg reached the end of the video and is just using the last frame it received until the audio ends). So I'm not sending enough frames to ffmpeg.
Below is a truncated version of the source code:
// Example code
int main()
{
// LoadAudio();
uint32_t frame_max = audio.sample_rate / 24; // 24 frames per second
uint32_t frame_counter = 0;
// InitializePipe2FFMPEG();
// (Left channel and right channel are always equal in size)
for (uint32_t i = 0; i < audio.left_channel.size(); ++i)
{
// UpdateWaveform4Image();
if (frame_counter % frame_max == 0)
{
// DrawImageAndSend2Pipe();
frame_counter = 1;
}
else
{
++frame_counter;
}
}
// FlushAndClosePipe();
return 0;
}
The commented-out functions are fake and irrelevant. I know this because "UpdateWaveform4Image()" updates the waveform used to generate the image every sample. (I know that's inefficient, but I'll worry about optimization later.) The waveform is a std::vector in which each element stores the y-coordinate of each sample. It has no effect on when the program will generate a new frame for the video.
Also ffmpeg is set to output 24 frames per second--trust me, I thought that was the problem too because by default ffmpeg outputs to 25 fps.
My line of thinking for the modulus check is that frame_counter is incremented every sample. frame_max equals 2000 because 48000 / 24 = 2000. I know the audio is clocked at 48kHz because I created the file myself. So it SHOULD generate a new image every 2000 samples.
Here is a link to the output video: [output]
Any advice would be helpful.
EDIT: Skip to 01:24 to see the waveform flatline.