Convert OGG to MP3

What is actually inside an .ogg

Ogg is a container, not a codec, and what it holds has changed over the years. An older .ogg almost certainly contains Vorbis — the free alternative to MP3 that games, Wikipedia and open-source software adopted precisely because MP3 was patented. A newer one may contain Opus, which is a considerably better codec than either and now carries most voice chat and a good deal of streaming. Both are decoded and both come out as MP3; which you started from affects how much the conversion costs.

Either way this is a lossy-to-lossy transcode, and both source codecs are more efficient than MP3 at the same bitrate — Opus dramatically so below 128 kbps. The practical rule is to encode a little above the source: a 96 kbps Opus file needs perhaps 160 or 192 kbps of MP3 to carry what it contains, because MP3 simply needs more bits to say the same thing. Matching the number rather than the quality is the mistake that makes transcoded audio sound worse than it should.

Speech recordings are where the gap shows most. Opus was built for voice and is remarkable at low bitrates; MP3 was built for music in 1993 and is poor below about 96 kbps on anything. A voice note converted at a matching bitrate will sound noticeably rougher than it did, and the fix is more bits rather than a different setting.

Vorbis and Opus both hold more than two channels, and MP3 does not, so a surround file is folded down to stereo. Opus also works natively at 48 kHz, which MP3 handles, so nothing is resampled in the common case. Tags do not currently come across, which matters for a game's audio folder rather less than for a music library.

When you want something else

Before converting, check whether the thing rejecting the file is as fussy as it looks. Every current browser, phone and computer plays both Vorbis and Opus, and VLC plays everything — the genuine cases are older car systems, some DAP players and specific editing software.

ffmpeg handles either codec and keeps the metadata: ffmpeg -i in.ogg -c:a libmp3lame -q:a 2 -map_metadata 0 out.mp3. If the destination can take AAC instead, -c:a aac -b:a 160k produces a better file at the same size, and almost everything that plays MP3 today also plays AAC — the exceptions are older than most people assume.