AAC is the codec that succeeded MP3, and it is everywhere: YouTube, Apple Music, most streaming services and most phone recordings use it. Usually it sits inside an M4A or MP4 file, but some recorders, radio streams, dashcams and download tools save it as a bare .aac file, and that is the version older players, car stereos and some editing software choke on. Converting to MP3 gives a file that every device plays. This page opens with MP3 as the output; drop the AAC files and convert. AAC is lossy and more efficient than MP3, so the MP3 needs a slightly higher bitrate to sound the same. If the AAC is 128 kbps, choosing 192 kbps for the MP3 avoids adding audible loss; going below the source bitrate will make things worse rather than smaller for free. If you are not sure what the original bitrate was, 192 kbps is a safe middle ground for music and 128 kbps is fine for speech. The files are processed one at a time in your browser, with nothing uploaded. If your files have the .m4a extension, they are the same AAC audio in a container, and M4A to MP3 Converter covers them. For audio inside a video file use MP4 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.