Can 9V 1A adapter be used for 12V 1A speakers?

No. If it works at all, it will be underpowered, and possibly damage the speakers.

To power 12V speakers, you have to have an adapter with EXACTLY 12V (no more, no less), and at least 1A. You could hook up a 12V 4A adapter if you wanted to, your speakers would just only draw 1A of power.

Make sure you use a 12V adapter though!!