For these builds, you need a modified driver. (Disclaimer: Use at your own risk. Proceed with caution.)
Symptom: A red light is glowing inside the headphone jack, and no sound comes from speakers. Cause: The optical audio switch is stuck. Fix: Apple Macbook Pro A1278 Audio Driver Windows 10
If the red light inside the headphone port remains turned on, the physical micro-switch inside the port is stuck in digital mode. Gently insert a headphone plug or a wooden toothpick into the jack a few times to physically release the internal spring switch. For these builds, you need a modified driver
A frequent issue on the A1278 is a persistent red light glowing from the 3.5mm headphone jack, which mutes the main internal speakers. Windows mistakenly believes a digital optical SPDIF cable is plugged in. Cause: The optical audio switch is stuck
Since finding the correct driver can be challenging due to the age of the machine, here are the most reliable sources:
Sometimes the driver is installed, but Windows 10 has clung to a generic driver that isn't compatible.