If you try to ship speakers overseas through Japan Post, you will find that the company does not accept speakers of any kind. Some speakers are magnet-shielded while others are not. The magnet-shielded ones are not considered dangerous goods when transported by air. However, it is not possible for Japan Post to check on whether each speaker is magnet-shielded. That is why the company will not accept any kind of speaker for international shipping.
Some people misunderstand the reason why Japan Post does not accept speakers. The reason is that magnets affect the airplanes’ electric devices. Therefore, speakers are considered dangerous goods.
Magnet-shielded speakers can be accepted for air transport if you provide some kind of proof that the model is magnet-shielded. Normally, the product’s URL on the manufacturer’s website or catalogue will do. Most speakers which are designed for use next to PC monitors or TV screens are magnet-shielded, while speakers for audio use (especially older speakers) are not.
You should have no problem transporting speakers by ocean. However, the minimum for ocean freight is one cubic meter. Therefore, this will perhaps be an option only when one is sending a large vintage speaker or other audio equipment (such as an amplifier) or when there is no other option.