Opus is the codec behind voice messages and voice calls in WhatsApp, Discord and most browser based calling, because it keeps speech clear at very low bitrates. When you export or save a WhatsApp voice note on Android it arrives as an .opus file, and most media players, editors and transcription services will not open it. Converting to MP3 fixes that. This page opens with MP3 as the output and 128 kbps selected. Voice messages are recorded at a low bitrate in the first place, so 128 kbps already keeps everything in the recording, and choosing higher only makes the file bigger without making it sound any better. Drop the .opus files and convert. A common reason for converting is to keep a voice note somewhere safe, attach it as evidence or a record, or edit it in an audio editor. Because the conversion runs in your browser, the content of the message never leaves your device, which matters for private conversations. If the file came from Telegram instead, it will usually have an .ogg extension, which the same converter reads. To turn a voice message into text use Speech to Text Audio Transcriber, and for Telegram files see OGG to MP3 Converter.
The split that matters most in audio formats is lossy against lossless. A lossless format such as WAV or FLAC stores enough information to reconstruct the original waveform exactly, which is why WAV files run around 10MB per minute of stereo CD quality audio. Lossy formats such as MP3, OGG Vorbis, AAC, and Opus use psychoacoustic models to discard sound that listeners are unlikely to perceive, typically hitting a tenth of the size for audio most people cannot distinguish in normal listening. The important consequence is that the discarded information is gone permanently. Converting MP3 to WAV inflates the file back to full size without recovering anything, and each further lossy conversion compounds the loss, an effect known as generation loss. The practical rule is to keep a lossless master and export lossy copies from it, rather than converting between lossy formats repeatedly. Bitrate is where quality and size are actually traded. At 128 kbps most listeners cannot reliably distinguish MP3 from the source on laptop speakers or earbuds, though the gap becomes audible on good headphones with dense music. At 256 to 320 kbps MP3 is effectively transparent for nearly everyone. OGG Vorbis reaches similar quality at a somewhat lower bitrate, because it is a newer design than MP3, and Opus does better still, which is why messaging apps and voice chat use it. Sample rate and bit depth apply to the uncompressed side. This tool writes WAV at 16 bit and 44.1 kHz, the CD standard, which is the format editing software and mastering workflows expect, and which every audio application on every platform can open without conversion.