no gui appears and this error message appears in the console:

[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: java.lang.NoClassDefFoundError: paulscode/sound/codecs/CodecIBXM
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at pl.asie.charset.module.audio.storage.AudioRecordThread.getCodec(AudioRecordThread.java:80)
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at pl.asie.charset.module.audio.storage.AudioRecordThread.addIfCodec(AudioRecordThread.java:46)
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at pl.asie.charset.module.audio.storage.AudioRecordThread.getSupportedExtensionList(AudioRecordThread.java:54)
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at pl.asie.charset.module.audio.storage.AudioRecordThread.getSupportedExtensions(AudioRecordThread.java:41)
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at pl.asie.charset.module.audio.storage.GuiRecordPlayer$DialogThread.run(GuiRecordPlayer.java:48)
[22:01:04] [Thread-31/INFO] [STDERR]: [java.lang.ThreadGroup:uncaughtException:-1]: at java.lang.Thread.run(Unknown Source)

The bug was specific to Forge 2851+, and is now fixed.

Something's wrong with your Forge installation - CodecIBXM should be bundled by it.

Oh weird, it is in my forge jar but the problem is persisting. My apologies. It may be some other mod interfering then, I'll have to do some more digging.