If one earbud is completely silent, check charging and reset (AirPods) or coupling (Galaxy Buds); if it is only quieter, check debris, ear tips and the balance setting. The order and model differences follow Apple and Samsung's official guides.
Bluetooth codecs move anywhere from 160kbps (LC3) to 990kbps (LDAC) — a sixfold spread. Measured against a 1,411kbps CD stream, SBC keeps 23% of the original and even LDAC's top mode keeps only 70%.
Bluetooth 5.4's new features — PAwR and Encrypted Advertising Data — were built for electronic shelf labels, not audio. What actually changes sound is LE Audio and the LC3 codec. Here is the real 5.3 vs 5.4 difference, straight from the official spec documents.