If you want to easily access the metadata for multiple .wav or .mp3 files, including the duration of each file, consider downloading the free utility MetaDigger. It's available here.
To access the metadata for multiple files simply go to File . . . Scan Files, and select the folder holding the audio and video files you need to process. As shown below the duration of each file will be listed in a table along with other basic metadata.
You can then go to File . . . Export Records , and the data will be exported to tab delimited text file which can be imported into Excel.