I'm able to screencast now, but after a while I get I read a lot, and one of the solutions seems to be in the increase of the ALSA Buffer size.

I searched a lot on how to do it but most ways aren't possible on Arch.

So how do I increase it?

Regards.

That is in the ALSA sources in alsa-audio.h, ALSA_BUFFER_SIZE_MAX. You have to obtain the source for ALSA, change this value, recompile it, and then install it.

No other trick?

Sorry, there are no shortcuts. This value predetermines the size of buffers and other resources allocated by ALSA and can only be altered in the source code before compilation. It would be inadvisable for ALSA to not take all possible needed resources when it initializes, because part of ALSA is a driver. If there isn't enough memory at ALSA driver start, then the driver fails and there will be no sound capabilities provided by ALSA. If there is enough memory, then whatever memory required by the ALSA driver is taken at the load time and kept so that those buffers are always available. Although, there have been cases; like yours, where the buffer sizes are still insufficient. In those cases, the only option is to modify the source and recompile.

Then I'll have to record with Audacity and after that join audio+video.