Did you try the following?
%pip install numpy
%pip install pydub
%pip show pydub
You want the special magic symbol and not an exclamation point.
Context
The magic pip command variation was added in 2019 to ensure the install occurs in the environment where the kernel is running that backs the active notebook.
The exclamation point in combination with install/ otherpip commands doesn’t do that and can lead to issues, much like you are seeing. You should be also using %pip list (or %pip show magic variation since you were trying with show) to see what is installed in the kernel environment that your notebook is using.
See more about the modern %pip install command here. Then for more background, the second paragraph here goes into more details about why the exclamation point may lead to issues.