Basically, the dialog box shows up the first time to select my mp3 file. It doesn't show up the second time and I cannot continue playing until I close Minecraft. I am using a Macbook Air (High Sierra 10.13.6). The dialog box shows up under Minecraft and not Finder. When I load the dialog the second time, it never shows and hitting "Show All Windows" only shows the game when it also would show the dialog if the dialog loaded.

I used a 10 minute disc as a test and I used Forge 1.12.2-14.23.5.2796! This happens in both fullscreen and windowed mode! Edit: Fullscreen just does it first time. No dialog shows up at all! Also, leaving the world and coming back (SSP) does not reset the variables to allow opening the dialog again!

I have the below mods installed with default settings:

AlbedoTorches-1.0.4.jar
AppleSkin-mc1.12-1.0.9.jar
Bedrock+Miner+1.12.2+by+XxRexRaptorxX+(v.1.0.4).jar
Charset-Audio-0.5.5.6.jar
Charset-Lib-0.5.5.6.jar
Clumps-3.1.1.jar
ColoredLights-1.12.2-v1.1.1.jar
DynamicLights-1.12.1.jar
Forgelin-1.8.2.jar
Galacticraft-Planets-1.12.2-4.0.1.184.jar
GalacticraftCore-1.12.2-4.0.1.184.jar
Hwyla-1.8.26-B41_1.12.2.jar
InventoryTweaks-1.63.jar
MCMultiPart-2.5.3.jar
MicdoodleCore-1.12.2-4.0.1.184.jar
NotEnoughCodecs-0.6.jar
OpenBlocks-1.12.2-1.8.jar
OpenComputers-MC1.12.2-1.7.3.146.jar
OpenModsLib-1.12.2-0.12.1.jar
OpenPrinter-1.12.2-0.1.0.2.jar
OpenPython-MC1.12.2-OC1.7-1.0.1.jar
OpenSecurity-MC1.12.2-1.0-13.jar
PeripheralsPlusOne-1.12.2-1.1-build-T58.jar
RedstoneRemote-1.12-1.1.0.jar
ShadowMC-1.12-3.8.0.jar
TheFramework-1.12.2-1.0.1-build-T23.jar
[1.12.2]+SecurityCraft+v1.8.10.jar
[1.12.2]Gender-1.2.9.jar
albedo-0.1.3.jar
albedocompat-1.0.jar
cc-tweaked-1.80pr1.11.jar
dmt-1.9.3.jar
guns2.jar
jei_1.12.2-4.13.1.225.jar
malisisblocks-1.12.2-6.1.0.jar
malisiscore-1.12.2-6.4.0.jar
malisisdoors-1.12.2-7.3.0.jar
malisisswitches-1.12.2-5.1.0.jar
mcjtylib-1.12-3.0.7.jar
notenoughwands-1.12-1.7.1.jar
ocsensors-1.0.4-b23.jar
plethora-1.12.2-1.1.13.jar
portality-1.12.2-1.2.2-14.jar
reauth-3.6.0.jar
redstonepaste-mc1.12-1.7.5.jar
torchmaster_1.12.2-1.6.4.62.jar
torchoptimizer-1.12.2-2.1.5.jar

It also happens when these are the only mods installed:

Charset-Audio-0.5.5.6.jar
Charset-Lib-0.5.5.6.jar
NotEnoughCodecs-0.6.jar

Set the profile to EXPERIMENTAL in charset.cfg. Artifacts of a bygone era...