Does eARC need a special HDMI cable?
AnswerMaybe
Usually not. eARC uses the HDMI Ethernet Channel wires, which every "High Speed HDMI with Ethernet" and every Ultra High Speed cable has. The bandwidth is small, about 37 Mbps, enough for lossless 7.1 audio and Dolby Atmos. Both the TV port and the soundbar or receiver must say eARC, and the cable must go into the TV's eARC input.
If eARC won't hand over: try the TV's dedicated eARC port, turn on HDMI-CEC on both devices, and replace very old cables without the Ethernet channel. Some older "High Speed" cables lack it. An Ultra High Speed cable is a safe fix.