how handle a path ($1)

I've:

/home/wms/mp3/

and

/home/wms/tmp/

Inside ~/tmp my script:

#!/bin/bash
br=64
for a in "$1"*.mp3 ;
do ffmpeg -i "$a" -ar 44100 -ab $br "$br""_tmp/${a%.*} ["$br"].mp3" ; done

I do:

/home/wms/tmp/./script.sh /home/wms/mp3/

How I handle $1?

because "@1" concides with /home/wms/mp3/first1

${a%.*} cuts extension file

but the path is not correct: this script runs only if I runs it inside ~/mp3