# MP4 subtitle generator: 110+ languages Generate subtitles from your local MP4 soundtrack in 110+ languages. Check the audio and a free preview, then export SRT, VTT or ASS. Your video stays on your device. ## MP4 subtitle facts Item | Result or policy --- | --- Tested audio | aac Detected container | mov,mp4,m4a,3gp,3g2,mj2 Subtitle targets | 110+ (118 currently listed) Output | SRT / VTT / ASS Video upload | No; only extracted audio is uploaded Account duration limit | 8 hours; guest limit 3 hours Full task price | USD $1/hour, rounded up to a cent ## Understand what an MP4 extension tells you MP4 is a container, not a single audio or video codec. Two files ending in .mp4 can hold different compression formats, stream layouts and metadata. The file picker accepts the extension, but the actual media inspection determines whether the soundtrack can be read. Renaming another file to MP4 does not convert its contents. For subtitles, the speaking track matters more than the picture encoding. A video whose image cannot play natively in your browser can still have readable audio. SubLunio shows subtitle text with timestamps when video playback is unavailable, so a missing picture in the browser is not automatically a reason to discard a valid soundtrack. ## Large MP4 files and the index at the end An MP4 can store its index near the beginning or near the end. Reading that index may require seeking far into the file before the first useful audio packet is found. SubLunio uses seekable local file reads instead of copying the entire movie into WebAssembly memory, which is important for files larger than 2 GB. This is not an unlimited-duration promise. The extracted MP3 needs temporary disk space, and account limits still apply to duration and upload size. Keep the browser open during local work. A file that was not finalized correctly, such as an interrupted recording with missing metadata, may require repair in its original recording workflow. ## Check alternate audio before treating a movie as single-language MP4 supports more than the simple one-video, one-audio layout that many users expect. A release may contain alternate languages, an audio description or multiple mixes. Inspect the track selector after choosing the file and listen to the intended track, especially when the filename does not explain its contents. Language tags can help, but missing or inaccurate tags are possible. The selected stream index identifies the actual audio used for extraction. If the track is silent, encrypted or damaged, changing the target subtitle language will not solve that input problem. Choose another valid speaking track or obtain an authorized, readable source. ## What our MP4 extraction check covers The attached local test uses an original twelve-second MP4 fixture with MPEG-4 video and AAC audio. Two tones appear at known positions, allowing the converted MP3 to be checked for both successful decoding and preserved timing. Chromium and Firefox passed this fixture with mono audio output at 16 kHz. That result verifies the listed container-and-audio combination, not every codec or protected MP4 in existence. It also does not measure speech-recognition accuracy: the synthetic test contains tones rather than dialogue. Your own preview remains the place to evaluate a real recording’s words and translation. The report and screenshot below make the scope of the compatibility check inspectable. ## Choose a language, then evaluate a preview Select the file above, choose its speaking track and pick the language you want to read. SubLunio offers 110+ subtitle target languages. Source speech recognition has a separate supported list and defaults to Auto; setting the interface language does not limit your target choice. A brief listen can prevent a commentary or alternate dub from becoming the source of an otherwise well-timed translation. The free web preview starts at the first subtitle and covers up to 5 continuous minutes. Compare the text with a clear line and a more difficult passage. You can start without an account, then sign in to retain the same valid task and continue with the audio already uploaded. A concise glossary can guide recurring names or specialist terms. ## Continue and save a portable subtitle Full generation is priced at USD $1 per hour of the entire video, rounded up to a cent. The preview portion is included in that full duration. Use account credit or pay for the task; card payments have a $1.00 minimum and unused credit remains available. The price is based on duration rather than the original video file size. Download SRT, VTT or ASS, choosing translated, original or bilingual text. Keep the subtitle beside the exact source edition and test dialogue at several points. The video stays local throughout extraction; server audio is temporary. Save your own subtitle copy and process only material you have the necessary rights to use. Automatic results still need review for names, meaning and timing. Actual MP4 extraction check: aac audio → MP3. Synthetic tones; this does not measure transcription quality. [Media test report](https://sublunio.com/static/media-evidence/chromium-extraction.json) ## Can a large MP4 be processed without loading the whole movie into RAM? The browser uses seekable chunk reads. Temporary audio storage and the configured duration limits still apply. ## Does MP4 always mean AAC audio? No. MP4 is a container; inspect the actual soundtrack codec and choose a valid audio stream. ## Why can my MP4 sound be processed when its picture will not play? Audio extraction and native browser video decoding are separate capabilities. ## Can a missing MP4 index be repaired by changing subtitle language? No. Use a complete, properly finalized media source. ## Keep exploring - [Compare supported video formats](https://sublunio.com/formats) - [Choose an audio track](https://sublunio.com/guides/video-audio-tracks) - [Process several files](https://sublunio.com/batch-subtitle-generator) Source page: https://sublunio.com/mp4-subtitle-generator Last updated: 2026-10-08