1. It's a different loudness
Media players, browsers and streaming platforms may change the level, and a small loudness difference changes how bright and punchy something seems. Compare at matched loudness before assuming the sound changed.
2. Master-bus processing didn't render as expected
Check the export range and that plug-ins on the master bus were active. Some plug-ins behave differently when rendering offline, and processing that depends on the playhead (like sidechain from a muted track) may not render as you expect.
3. The sample rate or bit depth changed
Exporting at a different sample rate than the session triggers conversion, and exporting at 16-bit without dither can add faint distortion on quiet tails. Export at the session rate, 24-bit or 32-bit float, and dither only for the final 16-bit version.
4. Lossy encoding
MP3 and AAC remove information and can raise peaks slightly, causing distortion on loud masters. Keep a lossless master, leave about −1 dBTP of true-peak headroom, and check the encoded file.
5. Different playback
Your DAW might output through your interface while the file plays through laptop speakers or Bluetooth — a completely different sound. Compare on the same device.
Quick checks: the audio analyzer shows format, loudness and peaks, and the LUFS meter compares loudness precisely.