Way of SL808x digital audio I/O less taxing than 2MHz PCM?

I need to exchange digital audio with Sierra Wireless SL808X from my microcontroller CPU. SL808X chips only seem to support 2MHz PCM which is very taxing on CPU. I only need to transfer voice, 128MHz sampling rate is a complete overkill. Isn’t there a way to input and output audio with lower sampling rate, via PCM pins, UART or whatever?