Drive a DC Motor with a WS2811 chip
If your motors are PMDC brushed ones you need pwm-output driver for each motor. Since WS2811 chips have constant current output you cannot use it for speed adjustment.
You may use PCA9685 (e.g.: https://www.adafruit.com/product/815) with output MOSFET stages. Because it's top pwm frequency is 1.6kHz you may hear switching noise. You can try to find similar PWM IO expanders with higher PWM frequency preferably higher than 20kHz.