Look for the Get It Free paragraph and link for a free version for non-commercial home use (might not show up on all web browsers or on a second visit). Simple: VideoPad - has an option under File menu, Add Images As Video option for importing image sequences (and select the folder containing the FireAlpaca image sequence).

Many other paid editors should also work well, although not all handle image sequences well - some import sequences as a slideshow by default, with about 5 or so seconds per image, not what you want for an animation. Personally, I am rather fond of Corel VideoStudio, a consumer-level paid product, has an import timelapse feature under the File menu (Insert Media) that works well with image sequences. To add sound to your animation, you are going to have to combine your frames into a true movie/video format (MP4, AVI, MOV, WMV, etc), using a video editor.

It is basically an image format with some multi-image features bolted on. There are a few web sites that try to play an animated GIF and a sound file simultaneously, with varying success.

No, the GIF format (from any program) cannot handle sound.

How to animate in firealpaca with music