Convert RM / RMVB to MP3 - extract audio
RealMedia audio is either Cook (later files) or one of the RealAudio speech codecs (older files) - both proprietary Real Networks codecs with essentially no consumer playback support today. Wave decodes both in your browser and re-encodes to a portable 192 kbps MP3 through libmp3lame. Audio codecs explained covers where MP3 sits vs. everything else.
What actually changes
RealMedia files hold Cook (RA10, the most common audio codec in RMVB from 2001 onward), RealAudio 8 (a music-quality codec that briefly competed with MP3), or one of the RealAudio speech codecs (ra_144 was low-bitrate speech at 8 kbps; ra_288 was the 16 kbps SIPR variant used for talk radio). Wave decodes all four through the on-device ffmpeg engine and re-encodes to MP3 through libmp3lame at 192 kbps VBR.
Because the video track is discarded entirely, the extract is fast - typically two-to-five times faster than a full video conversion on the same file. A one-hour RMVB movie extracts to an 85 MB MP3 in about 20 seconds on modern hardware. RealAudio speech-content sources are even faster - the codec runs at bandwidths so low that decoding is trivial.
RealAudio was the dominant streaming audio format from 1995 through about 2001. NPR, BBC, Voice of America, CBC, and hundreds of independent radio stations shipped RealAudio streams as their primary online-listening path through that entire window. A significant archive of preserved podcasts, radio talk-show recordings, and lecture captures from that era exists as .rm files with Cook or RA10 audio - and until now, playing them anywhere but RealPlayer has required a converter.
The MP3 output preserves whatever quality the source had. Cook at 20 kbps mono (a typical low-bandwidth radio stream) sounds like Cook at 20 kbps mono in the MP3; Cook at 96 kbps stereo (music-quality) preserves through as-is. If your source is a speech-optimised RealAudio Sipro file at 16 kbps, the MP3 will preserve every audible bit but stay bandwidth-limited - the source, not the target, sets the ceiling.
Three steps.
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Step 1
Drop the .rm or .rmvb file onto the card.
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Step 2
Pick MP3 as the output format.
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Step 3
Convert. Save the MP3.
Answers, specifically.
Does Cook audio decode cleanly?
Yes. The Cook decoder in ffmpeg-wasm is bit-exact against the reference. Wave gets clean PCM out of Cook and feeds it to LAME.
What if the source is speech-only RealAudio at 8 kbps?
Extracts to MP3 at 192 kbps VBR. The MP3 will not sound better than the source (that's not how re-encoding works), but every audible bit of the original recording is preserved.
What about early RealAudio at 28.8 kbps stereo?
Same treatment. Wave decodes through the appropriate RealAudio decoder and re-encodes to MP3.
RealMedia video with no audio track?
Wave rejects during inspect with a friendly message. Some silent .rm files exist (early experiments); those can't be extracted to MP3.
RMS files - are those the same?
RMS (RealMedia Secure) files carry Helix DRM. Wave can't decrypt them. Ordinary .rm / .rmvb files (the vast majority) are unencrypted and extract normally.
RealPlayer 10 captures - which era of files are those?
RealPlayer 10 shipped in 2004 and typically writes RV40 + Cook, which is the most modern RealMedia variant. All decoders handle it.
Chinese-drama and anime RMVB - audio quality?
RMVB audio was almost universally Cook at 64-128 kbps stereo, which is decent-quality for the pre-broadband era. The 192 kbps MP3 output preserves every audible detail.