Whilst it doesn't feature 'MAIN' or 'AUX' outputs, the NP-32 does have a shared Headphone/Stereo output which - if not being used for headphones - can be used to connect the keyboard to an external keyboard amp, PA system or monitor speaker (or, indeed, to an audio interface on a computer). (Plugging a cable into this output socket will mute the NP-32's onboard speakers.)
Because the NP-32 doesn't have a 'Mono' mode, it's important that the external system is also stereo. If it isn't, either one channel (left or right) will be ignored - leaving a fairly hollow sound - or the two channels will be combined which can result in some odd audio effects and artefacts.
A cable which on one end has a stereo-jack (1/4" TRS Jack) plug will be needed (the other end of the cable will depend on the equipment you're connecting the NP-32 to).
Alternatively, our PSR-EW400 is similarly a 76-note home keyboard but which also features professional Left/Mono and Right outputs for use with a PA system or similar. Additionally, the PSR-EW400 offers a greater range of sounds and has more advanced computer/iPad connectivity for recording, score-writing and production - see the link below.
Two standard instrument jack cables (such as guitar cables) can be used with the PSR-EW400, connecting either to a stereo channel on a mixer/PA or using two channels panned hard-left and hard-right. (Only one cable and mono channel are required to use the PSR-EW400 in mono mode.)