A 3GP file refers to a legacy video format made by the 3rd Generation Partnership Project for first-wave 3G phones, created when mobile hardware had scarce storage, sluggish CPUs, and low-capacity batteries, so it used a streamlined MP4-like container that emphasized efficiency and stable playback instead of clarity, holding compressed streams such as H.263 or early H.264 for video and AMR for speech, which leads to low-quality voices and very little ambient audio by modern expectations.

A common modern frustration with 3GP files is silent audio, caused not by damage but by AMR incompatibility, since players and browsers often omit AMR decoding for technical reasons and therefore play only the video; editors are even stricter and may refuse AMR completely, so it seems like the audio is missing when the software has deliberately left it out.

Another format, 3G2, often has more severe compatibility issues today, as its CDMA background means it carries audio codecs like EVRC, QCELP, or SMV that nearly no modern player supports, resulting in video-only playback until a converter decodes the old telecom audio and re-encodes it into AAC, confirming the file’s original use of a now-obsolete voice codec.

Unlike AVI and MKV, which are structurally distinct, 3GP and 3G2 stem from the same ISO Base Media File Format as MP4, so their layouts of atoms and boxes align closely, with the key difference being minor identifiers stored in the ftyp box—brands like 3gp4 or 3g2b—that many applications ignore.

Simply stated, 3GP and 3G2 were engineered for a different generation of technology focused on early phone playback, so modern problems like silent audio are predictable results of outdated codecs, and resolving them requires re-encoding the audio into a modern codec without altering the video.