The Music and Video apps might take some time to load files from an external memory card. If the files won't load after a few minutes, please reboot the device and try accessing the files again.
Please note that USB storage must be enabled, so you could use an external memory card successfully. You will find it under:
Settings -> Storage -> USB Storage
If you still face issues, you can reach out to our customer support via our contact form, on Twitter (@KaiHelps), or via Facebook. Our customer supports operates during the following business hours: Monday 07:00 a.m. - Saturday 07:00 a.m. EEST/GMT+3