newbie rtmp muxing question

[tasos]

Hello again.
I try to get a stream from rtmp , decode it , mux it  and finally save 
it to a file.
This is my first try https://pastebin.com/csNwT0x1 where i don't specify 
format=S16LE  and the program stops immediately after starting.
This is the second try https://pastebin.com/Q3djRm9K with format 
included, which seems to work BUT it doesn't save the file as expected.
So my two questions are
a)why do i have to specify s16le
b) why the filesink doesn't write to the file.

Thanks and sorry for the newbie questions :)

ps: this an ffprobe from my input stream

>  Duration: N/A, start: 0.000000, bitrate: 5251 kb/s
>     Stream #0:0: Video: h264 (Main), yuv420p, 1280x720 [SAR 1:1 DAR 
> 16:9], 5120 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
>     Stream #0:1: Audio: aac (LC), 48000 Hz, stereo, fltp, 131 kb/s