At the end of this video, you can see a magnetizer in action. Looks like it takes just a few seconds.

(1) This speaker has a long history || Altec Lansing 515 || DIY Completed Repairs Speaker - YouTube

Quick search, you can still buy one:

Medium Power Speaker Magnetizer-loudspeaker Magnetizer and Demagnetizer-Shengzhou Vatmag Tech Co., Ltd.-

Ron