macbook 13" mid 2010 model. sounds suddenly gone distorted

Not sure what the problem is, while as part of troubleshooting measure, you may

try startup (from off) to reset NVRAM or PRAM. See & precisely follow details here:

• Reset NVRAM or PRAM on your Mac - Apple Support

https://support.apple.com/en-us/HT204063

Another item that may help, try an SMC reset; helps with some hardware issues.

"If you experience issues with sleep, wake, power, charging your Mac

notebook battery, or other power-related symptoms, you might need

to reset the SMC (System Management Controller)."

Most likely one or both of the built-in speakers is bad. Adjust the balance slider in the Sound Output System Preferences to see if just one speaker is bad. If one speaker is good, then leave the slider on it although you will need to re-center the balance slider when using your headphones. I don't recall how difficult it is to replace the speakers on this model. I do know that removing the Bottom Case is difficult due to the rubber bottom cover that easily pulls off (especially if the Bottom Case was never replaced by Apple).