Are my files uploaded to a server?
No. The whole merge runs locally in your browser — your video and audio never leave your device, unlike upload-based merger sites.
Replace Audio in Video — Free Online, No Upload | CutFast
Replace a video's soundtrack with new audio free, right in your browser. Mute the original and swap in music or narration — no upload, no watermark, no signup.
Frequently asked questions
What happens to the video's original sound?
That depends on the mode. Add-audio and voiceover modes keep the original track and play the new audio alongside it; replace mode drops the original sound entirely and uses only the new track.
What if the audio and video are different lengths?
Each track keeps its natural length, and the output lasts as long as the longer one. If the audio is shorter, the rest of the video simply plays without it; if it's longer, the last frame holds while the audio finishes. Trim either file first for an exact fit.
Which formats can I merge?
Video: MP4, WebM, MOV, and MKV. Audio: MP3, WAV, M4A, AAC, and OGG. The result is always a single MP4 that plays on every major platform and social network.