Step 1: Understand the interval.
Given interval is
\[
\frac{3\pi}{4}\leq x\leq \pi
\]
Multiply by \(\frac{4}{\pi}\):
\[
3\leq \frac{4x}{\pi}\leq 4
\]
Therefore,
\[
\left[\frac{4x}{\pi}\right]=3
\]
for
\[
\frac{3\pi}{4}\leq x\lt \pi
\]
At \(x=\pi\), the value becomes \(4\), but a single point does not affect the value of the definite integral.
Step 2: Analyze \(\sin x\) on the interval.
For
\[
x\in \left[\frac{3\pi}{4},\pi\right],
\]
we have
\[
0\leq \sin x\leq \frac{1}{\sqrt{2}}
\]
Hence,
\[
3\leq 3+\sin x\lt 4
\]
Therefore,
\[
\left[3+\sin x\right]=3
\]
So,
\[
\left[\sin x+\left[\frac{4x}{\pi}\right]\right]=3
\]
Step 3: Evaluate the integral.
Thus,
\[
\int_{\frac{3\pi}{4}}^{\pi}
\left[\sin x+\left[\frac{4x}{\pi}\right]\right]\,dx
=
\int_{\frac{3\pi}{4}}^{\pi}3\,dx
\]
\[
=
3\left[x\right]_{\frac{3\pi}{4}}^{\pi}
\]
\[
=
3\left(\pi-\frac{3\pi}{4}\right)
\]
\[
=
3\cdot \frac{\pi}{4}
\]
\[
=
\frac{3\pi}{4}
\]
Step 4: Final conclusion.
Therefore,
\[
\boxed{\frac{3\pi}{4}}
\]