Wav source: playing speed and playing startup instant

Good morning, 

I'm a gnuradio beginner, and I'm trying to better understand some topics related to source and sink.

I implemented a very easy SW that allows me selecting a file from filesystem and then I provide the filename to the C++ source implementation, by calling the function:

   blocks::wavfile_source::make

The implementation works fine, but I have an issue.

Since sometimes I need to perform audio analysis on files that are quite long, I would like to have a way to use something like a timeslider to move on a specific time instant on the wav file, or at least to change playing speed of file. 

I found no property (looking both in C++ and Python generated code) to achieve this target.

Is there any way to perform this operation? 

Best regards