What Happens to Audio Tracks in a CineX Conversion: Which Survive, Which Become AAC

CineX judges every audio track on its own instead of treating a file's audio as one block. Tracks that both the device and the MP4 container accept — AAC (including the HE-AAC family, which shares the same codec name), AC3, E-AC3 and ALAC — are carried into the output MP4 unchanged, with no second compression. Another set is converted into AAC: most often DTS, DTS 5.1, TrueHD, FLAC, Opus and uncompressed PCM, and also the long-established MP3 and MP2, which a device can decode but MP4 has no recognised place to store. So "will my audio change?" is answered by the codec on that track, not by whether the file was converted.

Audio is the overlooked half of "my video will not play": when the picture is fine but a track cannot be read, the symptom is either total silence or a file that refuses to open at all.

1. Tracks that are carried over unchanged

The rule behind this group has two sides: the device recognises the codec, and MP4 has a defined place to store it. AAC is the most common audio format across phones, computers and streaming; AC3 and E-AC3 are usual in film material; ALAC belongs to Apple's own compressed format. Those tracks are copied, byte for byte, without being encoded again. Fail either side and a track cannot be copied - MP3 and MP2 decode fine on this device, but MP4 has no recognised place to store them, so they are not on this list.

A track the device can already read does not get re-encoded. That is not about saving time — it is about not making you pay for work that was never needed.

2. Tracks that become AAC

DTS and DTS 5.1, TrueHD, FLAC, Opus and PCM lack a direct read path on the playback side, and MP3 and MP2 fail on the container side, so CineX converts this whole set to AAC. This includes the lightly compressed and uncompressed ones: converting changes how they are stored, in exchange for a file that plays predictably everywhere.

That trade-off is deliberate. The first requirement is that the finished file plays and does not grow out of control. Leaving an unreadable track exactly as it is belongs to the most expensive failure there is: the conversion reports success, the file exists, and you discover the problem the moment you press play on a device.

3. Why tracks are judged individually

Within one file the situation can differ completely per track: the first may be DTS (must be converted) while the director commentary on the second is AC3 (can be carried as is). Converting everything because of one track would pay for work that was not required and would also re-encode tracks that needed nothing. The unit of judgement is therefore a single track, and one file can legitimately contain both "carry" and "convert" decisions.

A healthy default track does not mean every track is healthy. The judgement is made one track at a time.

4. What about multichannel audio

Multichannel tracks are within the range of inputs CineX has exercised on real devices with actual samples, including 5.1 and 7.1 material. What needs saying clearly: conversion puts the sound into a common, readable form. How many channels you actually hear, and over which output path, still depends on the playback device and whatever is plugged into it. Conversion does not change what the hardware can do.

5. How to check your own file first

Read the media information and go through the audio section line by line, watching three things: the codec name (decides whether it gets converted), the channel count (decides on which devices it means anything) and the language tag (decides what you see listed in a player). Neither the file name nor "does it have sound" gives you any of that.

If the codec you find is in this set, it is kept as is: AAC, AC3, E-AC3, ALAC. If it is in this one, it becomes AAC: DTS, TrueHD, FLAC, Opus, PCM, MP3, MP2.

6. How this connects to "will it play"

The container decides whether the system accepts the file, the video codec decides whether the picture must be recompressed, and the audio codec decides whether a track becomes AAC — three separate questions. Many "I converted it and it still will not play" cases end up being about audio rather than picture. The order of checks is in what to check first when a video will not open, and the picture side is explained in why some videos convert fast and others take longer.

7. Boundaries worth knowing in advance

• How a track is stored can change (DTS, TrueHD and FLAC become AAC). That is not a fault; it is the price of a universally playable file.
• Subtitles follow different rules: text subtitles (SRT, ASS) can be kept as tracks, while image-based PGS subtitles are read as text first and then carried over as a track.
• The work happens on your device; the video file itself does not need to be handed over somewhere else for processing.
• This article describes the judgement rules. What happens to each track in a particular file depends on the codecs it actually carries.

Frequently asked questions

Why did my second language track turn into AAC?

Because that track used a codec the device cannot read directly, such as DTS or TrueHD. Tracks are judged individually, so only the unreadable ones are converted while another AAC or AC3 track in the same file is carried over unchanged.

Does converting reduce the sound quality?

A re-encoded track is stored differently, so it cannot be described as identical to the source. CineX's trade-off favours a file that definitely plays over one that keeps an unreadable format intact.

Do single-track files go through the same judgement?

Yes. The unit of the decision is one track, and one track follows exactly the same rules as twenty.

Will any track be dropped?

Not because only the default one matters: every track in the file is processed, and what changes is how the unreadable ones are stored, not how many there are.

Axiom One LLC — CineX. Figures current as of 24 September 2026.