UART noise on audio lines
the TX line from the Arduino runs to the RX of the SA818


If you look closely at the block diagram for the SA818 (below)...



... you will see that the RX line is an output and this therefore means that you are connecting your UART output to the SA818 RX output. Even though you are doing this via resistor it doesn't make any sense at all.

Of course, I can't vouch for the datasheet being correct as it does appear to contradict itself later on when it says: -

 | 16 | RXD | RXD of the module and connect to external TXD |