I assume that you need to transfer data from UART to SRAM buffer. In this case you need not give a a bigger number in buffer. You can give 8bytes(SCB) or 4bytes(UDB) and transfer data based on the RX-FIFO On byte received and perform DMA operation with level trigger.No need to increment source address, increment destination address.
In case of transferring the data to I2S you need to use another DMA channel.Similar to above arrangement you need to transfer data 8 byte at a time. However we are not sure that this implementation would work with all datarate. Better to have a circular buffer similar to the USB-Audio example project.
We do not have an example project for this.