Need to join two videos and then convert

[vkosinets]

Scope:

I have two files of different format. The first one is wmv and the second one is mp4. All I need to do is to merge these files and then convert them to 3gp.
This should be done in linux command string.
I use ffmpeg software for this.

What I've already done:

At first step I tried to convert both files to 3gp with success.
Then I tried to convert both to avi and then merge.

I use the following command lines:

ffmpeg -i input.mp4 -f avi -s 352x288 -b 200 -r 25 - > temp.avi
ffmpeg -i promo.wmv -f avi -s 352x288 -b 200 -r 25 - >> temp.avi
ffmpeg -i temp.avi -acodec libamr_nb -s 352x288 -ar 8000 -b 120000 -vcodec h263 -ab 10.2k -ac 1 output.3gp

After this I've merged video file where the first part is ok, but in the second one I've awful sound.
The problem is not in the exact solution. I need to merge mov and flv, mp4 and 3pg, etc.
The question is how I should do this correctly. Probably I need to convert both video to some kind of raw data file with some format (also may be I need to set the same bitrate, framerate, etc). After this I'll be able to merge the files and then convert to format I need.

So the problem is more theoretical then practical.

Any help is appreciated!
Thanks.

[vkosinets]

Finally I've the following lines:

ffmpeg -i promo.mov -f avi -s 352x288 -r 29.97 -ar 44100 -b 120000 -ab 64k -ac 1 - > temp.avi
ffmpeg -i input.mp4 -f avi -s 352x288 -r 29.97 -ar 44100 -b 120000 -ab 64k -ac 1 - >> temp.avi
ffmpeg -i promo.wmv -f avi -s 352x288 -r 29.97 -ar 44100 -b 120000 -ab 64k -ac 1 - >> temp.avi
ffmpeg -i temp.avi -f avi merged.avi
ffmpeg -i merged.avi -vhook '/usr/local/lib/vhook/imlib2.so -x 0 -y 0 -i watermark.png' merged-watermark.avi
ffmpeg -i merged-watermark.avi -acodec libamr_nb -s 352x288 -ar 8000 -b 120000 -vcodec h263 -ab 10.2k -ac 1 output.3gp

This works fine.