Adding audio to a video means taking a separate audio file (music, voiceover, sound effects) and combining it with a video file into a single output. The result is a video that plays with your chosen audio instead of (or in addition to) whatever audio was in the original.
Replace versus overlay
Replace mode strips the existing audio and substitutes your new audio file. The original sound is completely gone. This is the right choice when the original audio is unwanted (background noise, private conversation, wind noise) and you want a clean replacement.
Overlay mode (also called mixing) keeps the original audio and adds your new audio on top. Both play simultaneously. This is useful for adding background music under existing narration, or layering sound effects over footage that already has ambient sound.
Most simple tools offer replace mode. Full mixing with volume balancing requires a more advanced editor.
Audio length versus video length
If your audio file is longer than the video, the audio is usually trimmed to match the video duration. The end of the audio is cut off.
If your audio file is shorter than the video, you have two options: the audio plays and then stops, leaving the rest of the video silent, or the audio loops to fill the entire video duration. Looping works well for background music but not for voiceovers or sound effects.
Audio format compatibility
Most tools accept MP3, WAV, AAC, and FLAC audio files. MP3 is the most universal and works everywhere. WAV is uncompressed and produces the highest quality but creates very large files. AAC is Apple's standard format and offers good quality at small file sizes. FLAC is lossless and produces excellent quality with moderate file sizes.
For adding background music to a social media video, MP3 at 192kbps or higher is perfectly adequate. The audio will be re-encoded during the merge regardless, so providing a very high bitrate source does not necessarily improve the final output.
Common use cases
Adding background music to silent screen recordings, product demos, or slideshow videos. Adding a voiceover to tutorial footage recorded without narration. Replacing wind-heavy outdoor audio with clean studio audio. Adding sound effects to short-form social media content.
How to add audio to a video in your browser
The MediaBrew audio tool lets you upload a video and an audio file, choose whether to replace the existing audio or let the tool handle the combination, and download the result. Audio is trimmed or looped to match the video length. Processing runs locally in your browser using FFmpeg WebAssembly. No upload, no account, no watermark.