Yes. Attach SRT/VTT as soft subtitles when the source exposes them. Burned-in subtitles are not supported by default.

Merge Audio & Video Online — MP4/MKV (No Re-Encode)

What is the Audio+Video Merger?

MP3stack combines separate video and audio streams into a single file. When codecs & container match, we mux (merge) with no re-encoding for original quality. If needed, we can transcode to MP4 (H.264/AAC) for maximum device compatibility.

Use only for your own content or when you have permission; follow each platform’s Terms. Availability and quality depend on the original source.

Need to grab sources first? Try YouTube to MP4, No Watermark Downloader, or Subtitles Downloader for SRT/VTT.

How to use

Paste a public video link or type keywords, then press Search.
Open Preview and pick a video quality (e.g., 1080p/1440p/4K) and the best audio track.
Choose Merge without re-encode if codecs fit the container.
If unsupported, select Transcode to MP4 (H.264/AAC) or keep original codecs in MKV.
Click Download to save the merged file.

For iOS/older TVs, MP4 (H.264/AAC) is the safest choice. For zero quality loss, use MKV when the video uses VP9/AV1+Opus.

What you can convert

YouTube — higher resolutions often split video+audio (DASH); we merge the best pair. See 4K Video Downloader.
Vimeo — creator uploads with separate tracks can be muxed when available.
Facebook / Instagram / TikTok — merge or transcode depending on the stream formats. Try Instagram Reels or TikTok to MP3 for sources.

Private, age-restricted, paywalled, or DRM-protected items are not supported.

Why choose MP3stack?

No quality loss when muxing compatible streams.
Device compatibility via MP4 (H.264/AAC) fallback.
Faster than re-encode — merging is near-instant vs full transcode.
Subtitle-friendly — attach SRT/VTT (soft) when exposed by the source.
Mobile-first UI — quick preview & selection.

Quick Start Guide

Paste link → Preview.
Pick best video + best audio.
Choose MP4 (transcode) or MKV (no re-encode).
Download the merged file.

On limited storage, prefer 1080p H.264; for edits/remixes, keep original codecs in MKV.

Features & Benefits

Container choice — MP4 or MKV depending on codecs.
Smart pairing — auto-suggests best video+audio by bitrate.
Optional transcode — H.264/AAC for broad compatibility.
Subtitles — attach external SRT/VTT when available (download subs).
Trim & merge — cut intro/outro before mux (when offered). See also Fast Converter.

Use Cases

Offline watching — merge 1080p video with high-bitrate audio.
Editing — keep original VP9/AV1 in MKV for lossless cuts later.
Presentations — ensure playback on any device via MP4.
Archival — store best video + best audio in one file.
Lectures & talks — also export audio with Lecture to MP3.

Device & Browser Guide

Android: Use Chrome; files save to Files → Downloads.

iPhone / iPad: Use Safari; MP4 (H.264/AAC) recommended; see Files → Downloads.

Windows / Mac: Chrome/Firefox recommended; Downloads folder.

Smart TV / Set-top box: MP4 is most compatible; MKV support varies by model.

Quality & Storage Planner

Format	Quality	Avg Size (4 min)	Best Use
MP4	1080p H.264 + AAC	~70–140 MB / 5 min	Broad device support
MKV	1080p VP9/AV1 + Opus	varies	No re-encode; original quality
MKV	1440p/4K VP9/AV1 + Opus	large	Highest detail when available

FAQ

What’s the difference between merging and transcoding?

Merging (muxing) combines streams without changing codecs—no quality loss. Transcoding re-encodes to a new codec/container for compatibility, which can affect quality and size.

When should I choose MP4 vs MKV?

Use MP4 (H.264/AAC) for maximum playback support (phones/TVs). Use MKV to keep original codecs (e.g., VP9/AV1+Opus) without re-encoding.

Why is there no audio after merging?

You may have chosen a video-only stream. In Preview, select a matching audio track or a quality that includes audio.

Does it support AV1/Opus?

Yes for MKV (no re-encode). For iOS/older TVs, transcode to MP4 (H.264/AAC).

Can I keep multiple audio tracks (e.g., commentary + main)?

When the source exposes multiple tracks and the container allows, pick and keep the primary. Multi-track keep is limited; for universal playback, prefer a single track.

Downmix 5.1 to stereo?

If you transcode to MP4 (H.264/AAC), stereo downmix may be applied depending on the selection shown.

Can I hard-burn subtitles?

Default is soft subtitles (SRT/VTT). Hard-burn is not enabled by default. You can download subs via Subtitles Downloader.

Join multiple clips into one?

This tool merges audio+video of a single item. For concatenation, prepare clips separately (same codec/params) then combine in an editor; or export MP3 via Fast Converter for quick audio workflows.

Is it legal and safe?

Use only for content you own or have permission to download; follow platform Terms and local laws.

Troubleshooting

Codec/container mismatch: choose MKV (no re-encode) or transcode to MP4.
Out-of-sync AV: re-merge with matching frame rate; avoid mixing streams from different sources.
Playback error on TV: transcode to MP4 (H.264/AAC).
Huge file: pick 1080p and moderate audio bitrate, or transcode with smaller settings.
Download didn’t start: allow pop-ups; click the final Download.

Copyright & compliance

All trademarks & logos belong to their owners. MP3stack does not permanently store files; items are generated on demand from public sources.

⚠ Attention:
By using this service you agree to our Terms of Service. Use it only for content you own rights to or that is free-licensed.

Merge Audio & Video (MP4/MKV)