The letters FX are almost pronounced as "effects". Especially in magazines such as ASM or Power Play game critics liked to use this term in the 80s as a synonym for the sound effects of a computer game. The FX were clearly distinct from the music. So with FX or sound effects one means e.g. explosion sounds in shoot'em ups, motor sounds at racing games etc. Whether you want to also count in samples, depends on how narrow you interpret the term or if a sample is part of a piece of music or independet from the music.
With the progress made in the 80s concerning the use of the SID chips the quality criteria of the game critics also rose. An FX that was still spectacular on the C64 in 1983, probably did not knock anybody off his feet in a game released in 1986.