WebM is not a synonym for Opus
WebM is a restricted form of the Matroska container. Its audio track can be Opus or Vorbis, and a video WebM can pair that audio with VP8 or VP9 pictures. Browser recorders often choose Opus; older files may use Vorbis. A player needs support for both the container and the codec inside it. Recognizing the .webm extension does not prove that a particular audio track can be decoded.
There is no packet-copy route to MP3
Opus and Vorbis packets cannot be placed unchanged into an MP3 file. PrivateMP3 decodes them and writes a new 192 kbps MP3. Because both WebM audio codecs are lossy, that is another lossy generation. A larger MP3 bitrate would not restore information removed when the WebM was recorded.
The other output buttons do different work, but neither preserves the original packets. M4A means a new AAC encode because a standards-compliant WebM does not contain AAC. WAV means decoded 16-bit PCM: useful for editing, much larger, and no better than the decoded source. Keeping the original WebM is the only way to keep its Opus or Vorbis bitstream untouched.
Voice recording and music
Speech can usually use a lower bitrate than music, especially for one mono voice. This page does not offer that choice; its MP3 output is fixed at 192 kbps. That setting favors compatibility, not the smallest possible long-form voice file. Music is more likely to expose damage from another lossy pass. If the destination accepts WebM, keep the original. If the music will be edited, use WAV as the working file and make one delivery encode after editing.
Was the recording finalized?
Play the original through to the end before converting. A recorder that crashed, a tab that closed before the recording stopped, or a partially copied file may be missing audio packets or duration information. No converter can recreate packets that were never saved. Also check that the capture contains an audio track; microphone or system audio may not have been enabled. A file merely renamed .webm may contain a codec outside the WebM rules, and a browser may parse the container while lacking a decoder for its audio.