You can create price alert here, copy coupons and view price history. What is best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Support A2DP music playback, bluetooth: 5.1, Rated power: 40W woofer + 15W full-range unit, Support IEEE 802.11 a / b / g / n protocol, Lighting: full-color ring galaxy atmosphere lighting effect, Frequency range: 45Hz ~ 20kHz, Wi-Fi: 2.4GHz/5GHz, Wired: 3.5mm AUX IN wired connection, Connect
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
1. ($291.99): BGAudio9
you should apply the coupon in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG1b5830, BGXIFD191, BGSaleFeb8, BGAudio3, BGAUGUST8
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Sound Pro 40W Bluetooth Speaker.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).