Addition 1.
([Pi] (4 c^3 ks^3 [Tau]^2 Sqrt[-2 I - [Tau] [Omega]] Sqrt[ 2 I - [Tau] [Omega]] + [Omega]^2 (4 + [Tau]^2 [Omega]^2)
(2 Sqrt[([Tau] (2 I - [Tau] [Omega]))/[Omega]] - I [Tau]^(3/2) Sqrt[[Omega] (2 I - [Tau] [Omega])] + 2 Sqrt[-(([Tau] (2 I + [Tau] [Omega]))/[Omega])] + I [Tau] [Omega] Sqrt[-(([Tau] (2 I + [Tau] [Omega]))/
[Omega])]) + c^2 ks^2 [Tau] [Omega] (4 I Sqrt[([Tau] (2 I - [Tau]
[Omega]))/[Omega]] + 4 [Tau]^(3/2) Sqrt[[Omega] (2 I - [Tau] [Omega])] - 4 I Sqrt[-(([Tau] (2 I + [Tau] [Omega]))/[Omega])] + 4 [Tau] [Omega] Sqrt[-(([Tau] (2 I + [Tau] [Omega]))/
[Omega])] + I [Tau]^(5/2) [Omega]^( 3/2) (Sqrt[-2 I - [Tau] [Omega]] - Sqrt[ 2 I - [Tau] [Omega]]))))/(8 c [Tau]^2 Sqrt[-2 I -
[Tau] [Omega]] Sqrt[ 2 I - [Tau] [Omega]] (c^4 ks^4 [Tau]^2 + 4 [Omega]^2 + 2 c^2 ks^2 [Tau]^2 [Omega]^2 + [Tau]^2 [Omega]^4))
Addition 2. In fact, the imaginary part is zero up to
FullSimplify[ ComplexExpand[ Im[(\[Pi] (4 c^3 ks^3 \[Tau]^2 Sqrt[-2 I - \[Tau] \[Omega]] Sqrt[ 2 I - \[Tau] \[Omega]] + \[Omega]^2 (4 + \[Tau]^2 \ \[Omega]^2) (2 Sqrt[(\[Tau] (2 I - \[Tau] \[Omega]))/\[Omega]] - I \[Tau]^(3/2) Sqrt[\[Omega] (2 I - \[Tau] \[Omega])] + 2 Sqrt[-((\[Tau] (2 I + \[Tau] \[Omega]))/\[Omega])] + I \[Tau] \[Omega] Sqrt[-((\[Tau] (2 I + \[Tau] \[Omega]))/\ \[Omega])]) + c^2 ks^2 \[Tau] \[Omega] (4 I Sqrt[(\[Tau] (2 I - \[Tau] \ \[Omega]))/\[Omega]] + 4 \[Tau]^(3/2) Sqrt[\[Omega] (2 I - \[Tau] \[Omega])] - 4 I Sqrt[-((\[Tau] (2 I + \[Tau] \[Omega]))/\[Omega])] + 4 \[Tau] \[Omega] Sqrt[-((\[Tau] (2 I + \[Tau] \[Omega]))/\ \[Omega])] + I \[Tau]^(5/2) \[Omega]^( 3/2) (Sqrt[-2 I - \[Tau] \[Omega]] - Sqrt[ 2 I - \[Tau] \[Omega]]))))/(8 c \[Tau]^2 Sqrt[-2 I - \ \[Tau] \[Omega]] Sqrt[ 2 I - \[Tau] \[Omega]] (c^4 ks^4 \[Tau]^2 + 4 \[Omega]^2 + 2 c^2 ks^2 \[Tau]^2 \[Omega]^2 + \[Tau]^2 \[Omega]^4))], TargetFunctions -> {Re, Im}], {ks > 0, \[Omega] > 0, \[Tau] > 0, c > 0}]
0