As far as I know, the voices on Yamaha keyboards usually have default volume settings (see the sixth paragraph below), and on some voices this can be rather low. There are three or four individual volume settings that you'll probably want to adjust as needed, and you must go into the Function menu to access them:
StyleVol (Style Volume) -- This sets the overall volume for the style playback. It does not control the volume for the individual tracks (channels) of the style. If you're using an external style file then you can use a style-editing utility to adjust the volume for the individual tracks within the style file, and on more expensive Yamaha arranger keyboards the volume for the individual tracks can be adjusted on the keyboard itself, but I know of no way to do this for the internal styles on the less-expensive keyboards.
M.Volume (Main Voice Volume) -- This sets the volume for the Main Voice.
D.Volume (Dual Voice Volume) -- This sets the volume for the Dual Voice if you're layering two voices (Dual On).
S.Volume (Split Voice Volume) -- This sets the volume for the Split Voice if you're splitting the keyboard (Split On).
Note that (as far as I know) most Yamaha keyboards have a feature called "Voice Set" which automatically sets most of the keyboard's functions to certain default values when you select a new Main Voice. On some keyboards there's an option to turn this Voice Set feature on or off, but on other keyboards there's no way to turn it off-- and the Owner's Manuals for those keyboards might not even mention the Voice Set feature at all! Whenever you select a Voice (i.e., the Main Voice), the keyboard will also set a particular Dual Voice to go with it, as well as a Reverb Type, Chorus Type, Harmony Type, and Arpeggio Type, including all parameters associated with these-- Main/Dual Voice Volume, Pan, Octave, Reverb Depth, Chorus Depth, Dual On/Off, Harmony On/Off, Arpeggio On/Off, etc. Most of the time the Split Voice will not be affected, although some keyboards may have "left arpeggio" voices that-- when selected-- will turn Split On and set the Split Voice Number and all Split Voice parameters.
Consequently, you should always select the desired Voice (Main Voice) first, then select the Dual/Split Voices, Reverb/Chorus/Harmony/Arpeggio Types, and make any desired changes to the various parameters associated with these. If the keyboard includes the Registration feature then you should save the settings to a Registration if you want to be able to recall them later.