FFmpeg can convert audio files to Opus format. Opus is playable in many web browsers and players in general. Opus is highly effective across a wide range of bitrates, from low (≪ 64kbps) for speech and high music bitrates.

ffmpeg -i in.wav -codec:a libopus out.opus

Typically it’s desired to set a specific output bitrate. The bitrate affects the audio quality–lower bitrate sounds more flat and mechanical.

ffmpeg -i in.wav -codec:a libopus  -b:a 32k out.opus

sets output bitrate to 32kbps, suitable for speech or lower quality music.