FourierTrigSeries[x,x,50]

Plot[%,{x,-2Pi,2Pi}]

FourierTrigSeries[Abs[x],x,20]

Plot[{%},{x,-3Pi,3Pi},AspectRatio->Automatic,PlotRange->{0,4}]

Plot[Pi/2 TriangleWave[(x-Pi/2)/(2Pi)]+Pi/2,{x,-3Pi,3Pi},AspectRatio->Automatic]

FourierTrigSeries[SquareWave[x/Pi],x,20]

Plot[%,{x,-2Pi,2Pi}]

Plot[SquareWave[x],{x,-2Pi,2Pi},ExclusionsStyle->Dotted,AspectRatio->Automatic]

f[t_]=2Pi(SawtoothWave[(t-Pi)/(2Pi)]-1/2)

Plot[f[t],{t,-Pi,Pi},AspectRatio->1]

Play[f[100  2  Pi t],{t,0,1}]

Play[2 Sin[100 2 Pi t],{t,0,1}]

Play[2 Sin[100 2 Pi t]-Sin[2 100 2 Pi t]+(2/3) Sin[3 100 2 Pi t]-(1/2) Sin[4 100 2 Pi t],{t,0,1}]

Play[2 Sin[100 2 Pi t]-Sin[2 100 2 Pi t]+(2/3) Sin[3 100 2 Pi t]-(1/2) Sin[4 100 2 Pi t]+(2/5) Sin[5 100 2 Pi t]-(1/3) Sin[6 100 2 Pi t]+(2/7) Sin[7 100 2 Pi t]-(1/4) Sin[8 100 2 Pi t],{t,0,1}]

Play[2 Sin[100 2 Pi t]-Sin[2 100 2 Pi t]+(2/3) Sin[3 100 2 Pi t]-(1/2) Sin[4 100 2 Pi t]+(2/5) Sin[5 100 2 Pi t]-(1/3) Sin[6 100 2 Pi t]+(2/7) Sin[7 100 2 Pi t]-(1/4) Sin[8 100 2 Pi t]+(2/9) Sin[9 100 2 Pi t]-(1/5) Sin[10 100 2 Pi t]+(2/11) Sin[11 100 2 Pi t]-(1/6) Sin[12 100 2 Pi t]+(2/13) Sin[13 100 2 Pi t]-(1/7) Sin[14 100 2 Pi t]+(2/15) Sin[15 100 2 Pi t]-(1/8) Sin[16 100 2 Pi t],{t,0,1}]