Interpolation of audio for new frames
When upsampling the number video frames to allow playing a video in slow motion, rather than for a frame rate increase for smoothness, you will likely need to modify the audio using a time-pitch stretching/shifting algorithm to stretch the audio out (increase the number of samples) for a longer play duration (to match the increase in slow motion video playback time), and to do so without distorting vocal or musical pitches.

If you just interpolate or resample audio without time-pitch correction, you will mess up all the pitch frequencies in the audio.

See this Wikipedia article for a description of several possible time-pitch stretching/scaling algorithms. There are multiple commercial and open source applications and libraries that can accomplish time-pitch modification/correction with varying degrees of quality and artifacts.