For editing mp3's under linux, I'd recommend sox. It has a simple to use trim effect that will do what you ask for (see man sox for datails - search (press/) for "trim start"). Example:
sox input.mp3 output.mp3 trim 1 5
You didn't mention it, but if your aim is just to remove the silence at the beginning of files, you will find silence effect much more useful (man sox, search for "above-periods")