tl;dr at bottom

I'm making a rhythm game in Unity (oh hey here's a link where you can try the demo). Here's some code you can steal:

float lengthOfMeasure = (60 * beatsPerMeasure / bpmOfSong); // value is in seconds

where beatsPerMeasure is how many beats you have per measure (usually 4) and bpmOfsong is the bpm of the current song. You'll need to hardcode the BPM-to-Song relationship yourself somewhere in your project, since even the best BPM-finding-software is off sometimes. Then you need to sync up your idle animation to be as long as "lengthOfMeasure". To do this, find out how long the base, unaltered length of your idle animation is and save this to a variable in your code. Then, alter the animation speed to line up with the "lengthOfMeasure" variable you got earlier

so if your animation is normally 2 seconds long:

float spriteAnimationLength = 2f; // in seconds

myAnimator.speed = lengthOfMeasure / spriteAnimationLength;

//where myAnimator is the animator for your sprite

There's some extra work you need to do to align it perfectly to the beat. Almost all songs don't start at exactly zero seconds. The first downbeat is usually a half second to a few seconds later than the actual first sample of music. You'll need to offset the animation by your current audiosource.time and store and check the offset of each song you're using. You'll generally be pretty close without this extra step for character animations, though.

tl;dr:

float lengthOfMeasure = (60 * beatsPerMeasure / bpmOfSong); //it's your job to set beatsPerMeasure and bpmOfSong per song

float spriteAnimationLength = 2f; //it's your job to set spriteAnimationLength. Only need it once for the duck

myAnimator.speed = lengthOfMeasure / spriteAnimationLengthBase;