Apple released Security Update 2016-004 today, along with iTunes 12.4.2 (and Safari 9.1.2, but this probably isn't the culprit)
It's unclear which caused the garbled & tinny sound, but if you experience this, fix it this way:
Go to /Applications/Utilities/Audio MIDI Setup.app
It should open to the Audio Devices window. If not, open it from the Window menu, or via Command-1.
Your selected sound output should be pre-selected in the lefthand pane. If not, pick your output. Usual choices are "Built-in Output" (internal speaker), "Built-in Line Output" (analog out), or "Built-in Digital Output" (toslink). Other choices may be present if you have a sound card.
The output format will have been reset to Format: 16000.0 Hz. Change this output to one of the offered choices, usually 44100 Hz or 48000 Hz. Your sound will return to normal.
Afterward, the "16000 Hz" sampling rate will have vanished from the pop-up menu. It was a nonstandard choice, which I've had happen several times after updates. Most updates don't gum up the audio, so I don't know what the exact cause is. My system uses digital out (toslink) to a receiver. The receiver was off when the updates were applied.