Hello Ian and welcome to the forum!

If you do not have it already, +++ HERE +++ is a link to the Korg EK-50 keyboard user manual.

The EK-50 has two USB ports. Port A accepts a USB memory stick that can be used for extra storage. Port B passes MIDI data between the EK-50 and a computer. Neither Port A or Port B supports audio over USB.

EK-50 audio would travel from the Output L & R jacks, through an audio interface, into your computer and into RealBand.

So the audio you're hearing is either coming from the MIDI synth associated with RealBand or from the speakers of your EK-50.

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Partial EK-50 Specifications Audio jacks and MIDI highlighted.
Jim Fogle - 2025 BiaB (Build 1128) RB (Build 5) - Ultra+ PAK
DAWs: Cakewalk Sonar - Standalone: Zoom MRS-8
Laptop: i3 Win 10, 8GB ram 500GB HDD
Desktop: i7 Win 11, 12GB ram 256GB SSD, 4 TB HDD
Music at: https://fogle622.wix.com/fogle622-audio-home