fix: resample audio when sample rates differ in merge_audio

[Mr-Neutr0n]

Summary

• The merge_audio() function in audio-chatgpt.py concatenates two audio signals without checking if they have matching sample rates. When sr_1 != sr_2, the first audio segment is written at the wrong sample rate (sr_2), causing it to play back at the wrong speed and pitch, producing corrupted audio output.
• Added a sample rate check that resamples signal_1 to match sr_2 using numpy linear interpolation before concatenation. This requires no additional dependencies since numpy is already imported.

Details

Before: sr_1 was read from the first audio file but never used or validated. The merged output always used sr_2, so if the two files had different sample rates, the first segment would be distorted.

After: If sr_1 != sr_2, the first signal is resampled to sr_2 using np.interp with linearly spaced indices, preserving the original duration while matching the target sample rate.

Test plan

• Merge two audio files with the same sample rate and verify output is unchanged
• Merge two audio files with different sample rates (e.g., 16kHz and 44.1kHz) and verify both segments play at correct speed/pitch
• Verify the output WAV file has the correct sample rate in its header