To do any of this, open the recording in the converter. The File check panel under the waveform shows the audio track's codec and its measured bitrate, and those two numbers decide most of what follows.
Start from what the recording already holds
A screen or meeting recording stores its sound compressed, usually as AAC inside an MP4 or MOV. Converting it cannot make it sound better, and there are only three ways to make the audio file smaller: keep the compressed packets as they are, encode fewer seconds, or encode at a lower bitrate. PrivateMP3 offers the first two. Its MP3 and AAC encodes both run at one fixed rate, 192 kbps, and there is no lower setting, so a recording that is already below 192 kbps gets larger when it is encoded again, not smaller.
A mono microphone track does not change that. The converter writes MP3 at the same 192 kbps for one channel as for two; on the two-track sample recording, 20 seconds of the mono track saved as 481,128 bytes and the stereo track as 481,704 bytes. Only WAV halves for mono, because WAV size is set by channels, sample rate and duration.
Measured: a 10-minute recording, whole and with breaks removed
The source is a generated test recording, not a real lecture: a 10:00 QuickTime movie of 728,935,720 bytes with a 1080p test picture and one AAC stereo audio track at 128 kbps and 48 kHz. It was opened in the converter in desktop Chrome on 15 October 2026 and each output was saved and weighed. For the second half of the table, two breaks were removed with Remove selection, 2:00 to 3:30 and 6:00 to 7:00, leaving 7:30.
| Output | Length | What the converter did | Saved size | Time |
|---|---|---|---|---|
| M4A | 10:00 | Copied the AAC packets unchanged | 9,739,957 bytes | 0.8 s |
| MP3 | 10:00 | Decoded, encoded at 192 kbps | 14,400,744 bytes | 10.3 s |
| WAV | 10:00 | Decoded, 16-bit PCM | 115,200,044 bytes | 5.2 s |
| M4A | 7:30 | Decoded, encoded as AAC at 192 kbps | 10,888,850 bytes | 3.4 s |
| MP3 | 7:30 | Decoded, encoded at 192 kbps | 10,800,744 bytes | 8.6 s |
Removing a quarter of the recording made the M4A 1,148,893 bytes larger, about 12 percent. The cut M4A and the cut MP3 are both bigger than the uncut copy. The copy is the only path that keeps the source's 128 kbps, and any cut, volume change or fade rules it out, because the converter does not retime compressed packets across a join; it decodes and encodes instead. The File check panel says this before you press Convert: after the first cut, its export line changes from the copy to an AAC encode at 192 kbps.
The rule: when removing time saves space
An encode at 192 kbps writes 24,000 bytes per second of output. The copy writes the source's own rate, for a 128 kbps track 16,000 bytes per second, plus about 188 bytes per second of MP4 index. So cutting saves space only when the share of the recording you keep is smaller than the source bitrate divided by 192.
- 128 kbps source: keep less than two thirds. Removing 2:30 of 10:00 kept 75 percent, which is why the result grew.
- 160 kbps source: keep less than about 83 percent, so removing more than a sixth starts to pay.
- 96 kbps source: keep less than half.
- 192 kbps or above: any cut saves space, and the uncut copy is already as large as the MP3 would be.
If the only reason for cutting was size, check this first. If the breaks have to go anyway, for listening, the extra bytes are the price of the edit, and MP3 and the AAC encode cost about the same.
Fitting a size limit
Many upload forms and mail services cap a file. Gmail, for example, documents a 25 MB attachment limit for personal accounts and turns anything larger into a Drive link (Google: attachment size limits). At the measured rates, the longest stretch that fits under 25 MB is about 25:44 as an unchanged 128 kbps copy and 17:21 as a 192 kbps MP3. An hour is about 58 MB as that copy and 86.4 MB as MP3, so an hour does not fit in either form.
The way to fit it is to split, and splitting is a cut, so each part is encoded at 192 kbps whatever the source rate. Choose A size limit in File check and enter the limit; the panel names the smallest output that fits, or how many parts the recording needs and how long each may be. Then select the first part, press Keep only selection, convert and save, and repeat for the next part. When this was tested for a 5 MB limit, the panel proposed parts of 3:28, and a saved part weighed 4,992,936 bytes. The audio file size calculator gives the same arithmetic for any length without opening a file.
A working order
- Open the recording and read the audio line in File check: codec and measured kbps.
- If it is AAC and you only need it smaller than the video, save the unchanged M4A copy first. It takes seconds and is usually the smallest file you can get here.
- If you need to remove parts, decide with the rule above whether it will shrink the file or only shorten it.
- If a limit still applies, split with Keep only selection, one part per export.
- Keep the original recording until each saved part has been played through.
None of this sends the recording anywhere. It is read from your device in ranges and the saved audio is written in the browser; the guide to conversion without uploading explains how to check that in the browser's network panel.