What about using a hash?
sl = {"sample_loop_01" => :loop_amen,"sample_loop_02" => :loop_garzul}
define :play_sample do | number |
sampleToPlay = "sample_loop_" + number.to_s
x= sampleToPlay.to_s
sample sl[x]
end
play_sample("01")
Essentially it carries out the substitution that you want.