f[freq_?NumericQ,amps_List]:=Function[t,Sum[amps[[n]] Cos[2 Pi n freq t],{n,1,Length[amps]}]]

amps:={1,.01,.3,0.01,.2,0.01, .05, .01}

freqmiddleC=440/2^(9/12);
freqCSharp=440/2^(8/12);
freqD=440/2^(7/12);
freqDSharp=440/2^(6/12);
freqE=440/2^(5/12);
freqF=440/2^(4/12);
freqFSharp=440/2^(3/12);
freqG=440/2^(2/12);
freqGSharp=440/2^(1/12);
freqA=440;
freqASharp=440*2^(1/12);
freqB=440*2^(2/12);
freqhighC=440*2^(3/12);

note[t_]=f[freqmiddleC,amps][t]
Plot[note[t],{t,0,2 Pi/100}]
Play[note[t],{t,0,2}]

chord[t_]=f[freqmiddleC,amps][t]+f[freqE,amps][t]+f[freqG,amps][t]
Plot[chord[t],{t,0,2 Pi/60}]
Play[chord[t],{t,0,2}]