Video and audio conversion: an extension does not guarantee playback
One MP4 file may play on a device while another with the same extension does not. The filename tells only part of the story: the included tracks and their encoding also determine compatibility.
The box and its contents
A container such as MP4 or WebM holds tracks and metadata; a codec describes how a track is encoded. A file can contain audio and video or no audio at all. Renaming its extension neither transcodes the content nor fixes incompatibility.
References: MDN — Media Container Formats
Extracting audio does not improve the recording
When listening is the goal, an audio file avoids carrying the video track. MP3 is lossy. WAV can contain different encodings and commonly carries uncompressed PCM. Making a poor source larger cannot recover discarded sound detail.
Choose a format the receiving application accepts. Keep the original for editing and listen to the output’s beginning, ending, quiet passages and loud passages. File size is not a quality score.
References: MDN — Web Audio Codec Guide · MDN — Media Container Formats
A practical video and subtitle check
Play the beginning, middle and end. Match a visible action or speech with its sound; check for unexpected cropping, stretching or rotation. Converting a source without audio cannot create its missing audio track.
For external subtitles, inspect an early cue and a later one. A fixed mismatch may respond to a timing offset; steadily increasing drift cannot be corrected by one fixed offset alone. Only use and distribute media you have the right to use.
A size estimate for 90 seconds of audio
Constant-rate audio at 128 kbit/s uses approximately 128,000 × 90 ÷ 8 = 1,440,000 bytes, or 1.44 decimal MB, before overhead. Variable bitrate makes this an estimate rather than a promised file size.
For an arithmetic comparison, 48,000-sample/second, 16-bit, two-channel PCM needs 17,280,000 audio-data bytes for the same duration, before a file header. A larger output does not establish that lost source quality was recovered.