This allows you to confgure the mixing levels of the emulated audio hardware. This option is provided because the mixing levels could vary on different types of hardware (such as different models of the Famicom or different versions of game carts), it allows you to fine tune the audio emulation against a specific set of hardware.
Before changing these settings, please be aware that mixing levels are not stored in the NSF format and that your song could be played back with any configuration of mixing levels!
APU1 is the first audio out pin (pulse 1 & 2) and APU2 is the second audio out pin (triangle, noise & DPCM), the rest are the expansion chips. The sliders ranges from -12 dB to +12 dB, the default value is 0 dB for all sliders.