Is there a way to set different sampling frequencies to channels in ADS1256?
The ADS1256 has a single ADC that is multiplexed between all channels.
If you read from a single channel continuously, then you can get 30000 samples per second out of it.
Since switching channels takes a couple of commands and some settling time before you get valid data, the total throughput drops when you start switching channels.
Page 21 of the datasheet shows the effective sampling rates when using all channels:

The datasheet doesn't explicitly describe what happens if you constantly change the sampling rate, but it will probably result in a total sampling rate slower than you get just from cycling through all the channels.

This is not a good chip if you want to sample audio at a high rate on one channel while sampling sensor data at a slow rate on the other channels.
Either use a different chip, or use two ADS1256 - one for your audio and the second for the sensors.