Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (291.99 USD): BGAudio9
(CouponsFromChina.com tip: apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BG1b5830, BGXIFD191, BGSaleFeb8, BGAudio3, BGAUGUST8
(probably won’t work, but you can try them out)
Price history and price alert
Price history (for all/most countries) for Xiaomi Sound Pro 40W Bluetooth Speaker is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).