UART mode is completely manual - only 'button mode' does the 'random/next/latch' behaviors. that's because if you are controlling via UART you can do whatever you like :)
For reasons we haven't nailed down yet, Trigger pins #9 and #10 require more current to trigger (they are set to inputs in the firmware so that's not it) and might not be possible to do so from a logic output device. Using a switch ought to work!
That's right, after it is done playing the sound it will check the buttons and find that the one you have held down is pressed so it will play it again - this is normal!
If you want to have it play a sound only once through, even if the button is held down, you can try making a sound with very long empty noise at the end, then use TnnHOLD mode so that it will play an empty noise. Then when the button is released and re-pressed it will start at the beginning. OGG type is best for this since it is compressed
The microconroller on board has to look through the entire filesystem every time it detects a button press, so the key to making loops short is keeping the filesystem clean and small.
Once you have your setup the way you want, make sure the SD card is empty, even reformat it with the 'blank' image if you've copy n pasted files around! Use WAV format not OGG because WAV plays faster. Also, make sure you are using T00 slot since that is the first button checked.
If you hear this, it is normal. The board does not include any anti-popping circuitry to prevent this initial transient.