Ok, first, to locate the problem. Open the recording (.m4a) in any other player. If speed is normal, then, it's Sound Recorder app probelm. Go to Settings, Apps, Installed apps, find Sound Recorder app, "...", advanced, repair it. If there is still a problem, Reset.
If .m4a recording speed sounds incorrect in other players too, than Settings, System, Sound, click on microphone you are using, properties. Select 2 channels, 16bit, 48000 (or 1 channel if that's an only option). If problem persists, turn off audio enhacements. Also, you can try updating microphone driver.