Well..... Garbled is already the past tense. So There really is not past tense for garbled.

Here is as a sentence with the word garbled. Nancy began talking with her friend on the phone but eventually her friend's voice sounded garbled.

The telemarketers words became garbled due to all of the background noise in my vicinity.

it is spoil

The frequency of the unwanted electrical signals mismatch with the sound and heard as "Noise" - a garbled form of sound.

The root word of garbled is garble, which comes from the Middle French word "garbeler", meaning to sift or sort out impurities.