Mpeg Streamclip audio issues after joining more than 2 files

hi community, I really need some help decluttering my collection of video files. The problem is audio when joing more than 2 files.

I'm a simple minded individual trying to join some wmv files into one file. After exporting to mpeg4 audio works when joining 2 files together but when I try joining 3, the exported file has audio for clips1st and 2nd but by the 3rd clip no sound? what I normal do is add A to app- copy then add B and paste then select all copy then add C which results ina pop up "do you want to open another stream" -open, then select all export to mpeg-4 using h.264, 50% quality sound: mpeg-4 aac, stereo, unscaled, interlaced scaling and reinterlace chroma and thats it. btw I don't have a clue what to do with the settings so I just trial and error with files of varying sizes but the sound issue is a puzzle.

When I playback the joined file on mpeg streamclip the sound works fine but when i export it, there is no sound during the 3rd clip.

please help me as i have alot of movies cut into 3 files which i would like as one file.

ps Also I dont know what i'm doing so i may need more step by step help.