Components:

Teensy LC without pins
https://www.mouser.com/ProductDetail/Adafruit/2419?qs=GURawfaeGuBJGDioriOxhQ%3D%3D

+ TVS multi-channel protector (through hole DIP)\
https://www.mouser.com/ProductDetail/Littelfuse/SP720APP?qs=RnOBYWd%2Fz7ctWfM2SrWSeA%3D%3D
https://www.mouser.com/datasheet/2/240/Littelfuse_TVS_Diode_Array_SP720_Datasheet.pdf-777227.pdf

DIP socket for TVS (option to unplug?)
https://www.mouser.com/ProductDetail/Preci-dip/110-83-316-41-001101?qs=FtMuP6KVi2QRJ6ZzYGl5hQ%3D%3D

+ Terminal block for connecting 9 wires to board (26-14 AWG)
https://www.mouser.com/datasheet/2/670/tb006_508-2306877.pdf
https://www.mouser.com/ProductDetail/CUI-Devices/TB006-508-09BE?qs=sGAEpiMZZMvPvGwLNS671%2FDanv8Jav06TCsCx7rFBbVQCZEBfEZ9%2Fg%3D%3D

Terminal block 3 position
https://www.mouser.com/ProductDetail/CUI-Devices/TB006-508-03BE?qs=vLWxofP3U2zmvvLYJBQLMA%3D%3D

+ 100 ohm 3-resistor array (3x)
https://www.mouser.com/ProductDetail/Bourns/4606X-102-101LF?qs=BKfsZh40Pd9uHJ2FCrpsTA%3D%3D

10k ohm 3-resistor array
https://www.mouser.com/ProductDetail/Bourns/4606X-102-103LF?qs=Qv7qbSW4mEEcSKVyNH8uMA%3D%3D

+ 470 ohm 3-resistor array (3x)
https://www.mouser.com/ProductDetail/Bourns/4606X-102-471LF?qs=6yLoizYthp4TRYKX8lkO3g%3D%3D

+ Small green LED indicator light (9x)
https://www.mouser.com/ProductDetail/Broadcom-Avago/HLMP-3507?qs=jT9z6tsiFNnJCBbLqZiZSA%3D%3D

+ 220 uF electrolytic capacitor for isolation
https://www.mouser.com/ProductDetail/KEMET/A759EK227M1AAAE018?qs=sGAEpiMZZMsh%252B1woXyUXj%2FgcXCpI1SPdRVcd4wgX6pE%3D

1uF film capacitor for audio in/out
https://www.mouser.com/ProductDetail/KEMET/R82DC4100AA60K?qs=Jv4FAWB%252B0HY7sWoVYON7mg%3D%3D

Headphone jack 3.5mm (2x)
https://www.mouser.com/ProductDetail/CUI-Devices/SJ1-3513N?qs=WyjlAZoYn501Uyi9N5Ym3w%3D%3D

Pushbutton locking switch for mode control (2 or 3x)
https://www.mouser.com/ProductDetail/Apem/MHPS2285?qs=%2Fs1LKTamv0wBYywlfe0WMg%3D%3D

4 channel op-amp for audio mixing
https://www.mouser.com/ProductDetail/Texas-Instruments/LM324AN-NOPB?qs=sGAEpiMZZMtOXy69nW9rM4ua1u5LNaSPMCRia0p6mAg%3D

10k ohm rotary potentiometer for audio signal volume
https://www.mouser.com/ProductDetail/Bourns/PTV09A-4015U-B103?qs=sGAEpiMZZMu7u4aXTvZ%252Bj1cug5VOOaxOVJ3SkWVGg5o%3D

10k ohm single resistor 0.1% (10x)
https://www.mouser.com/ProductDetail/Vishay-Dale/RN55C1002BB14?qs=sGAEpiMZZMsPqMdJzcrNwipD6ikct%252B19zQkR6nplfC4%3D

4.7V Zener diode protection
https://www.mouser.com/ProductDetail/onsemi/1N5917BRLG?qs=y2kkmE52mdNfdKQIlNN8jQ%3D%3D

Pin header 10pos (3x)
https://www.mouser.com/ProductDetail/Amphenol-FCI/68422-410HLF?qs=ws7IzJoRLdFBd%252BGwSXZbkA%3D%3D

Optional Lights power connector
https://www.digikey.com/en/products/detail/cui-devices/PJ-002AH/408446

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1uF capacitors for audio and bypassing (arrays not available)
https://www.mouser.com/ProductDetail/TDK/FK18X5R1E105K?qs=sGAEpiMZZMsh%252B1woXyUXj4dXoCGhcWKpcJXuHJFcIfE%3D

2-pin spring loaded connectors with mounting holes
https://www.mouser.com/ProductDetail/Mill-Max/858-22-002-10-001101?qs=BZBei1rCqCDCRgjqDs05TA%3D%3D

100 ohm 5-resistor array for ESD protection on inputs (2x)
https://www.mouser.com/ProductDetail/Bourns/4610X-102-101LF?qs=DNfJUgYH4CjbVaYwxkURpA%3D%3D

470 ohm 5-resistor array for LED indicators (2x)
https://www.mouser.com/ProductDetail/Bourns/4610X-102-471LF?qs=tbLXno17%252BNTSWajwQZb3HQ%3D%3D

10k ohm single resistor 1%
https://www.mouser.com/ProductDetail/Vishay-Dale/RN55D1002FB14?qs=sGAEpiMZZMsPqMdJzcrNwvkdYAxQ3%2Fr%2FJ8fqeOT96Zw%3D

10k ohm 5-resistor array for audio dividers (2x) - these are 2% tolerance, 1% and 0.1% tolerance below
https://www.mouser.com/ProductDetail/Bourns/4610X-102-103LF?qs=%252BG4UmwsSIwMvcNIi11TIkw%3D%3D
https://www.mouser.com/ProductDetail/Bourns/4310R-102-103FLF?qs=6IKrwJzvu5yb1rzVWSTKjA%3D%3D
https://www.mouser.com/ProductDetail/Vishay-Thin-Film/VTF158UF?qs=vt4di4biQt82M58vjTEcTg%3D%3D

100k ohm 2-resistor array for internal audio voltage divider
https://www.mouser.com/ProductDetail/Bourns/4604X-102-104LF?qs=cng4rebUNRl3VutXrhuzdA%3D%3D

Pin headers for mounting teensy in a removable manner (representative - check actual pin count)
https://www.mouser.com/ProductDetail/Amphenol-FCI/68422-410HLF?qs=ws7IzJoRLdFBd%252BGwSXZbkA%3D%3D
https://www.mouser.com/ProductDetail/Wurth-Elektronik/61301011821?qs=W%252B2sBeLta1ZhlLR0YCzonQ%3D%3D

4p4c (telephone jack) connector possibility
https://www.mouser.com/ProductDetail/CUI-Devices/CRJ015-1-TH?qs=iLbezkQI%252Bsi9L6%2Fu1tb6rQ%3D%3D