Built into Multisim is a microphone component but this requires the connection of a real microphone to your computer and will record the signal it receives. There are no native options for importing a WAV file and using the signal within Multisim.
One option that has been used to achieve this is a virtual audio cable. This is a Windows driver that allows the transfer of audio streams from one application to another. This would allow you to use a program for reading the file and transfer this into Multisim. However this requires the installation and parallel running of a third party utility. (see Discussion Forums link)
A preferred method for achieving this is in the creation of a custom instrument to handle the file. A good example of a LabVIEW custom instrument for the import of a WAV file has been posted on the NI Community
In Multisim 14 the community LabVIEW custom instrument is not supported, however it is possible to play arbitrary waveform data saved in CSV format in Multisim using a VirtualBench Source. One way to convert a WAV file to CSV format is to use a free piece of software called Audacity.