when i go to work at 6.00 am it is kinda cold out and my car turns over a little slow,not sure if it is the battery or the starter. it is a 2006 impreza with a 2.5l engine. once it warms up it does just fine. and now that my ecu has been replaced my cruise control slows me down like it is suppose to. also the aftermarket radio(jensen) is messed up every time i turn on the car i  have to turn the volume back up does that have something to do with the battery or could it be the alternator? it is under warranty so i am not sure if i can check it without voiding it.

Its very likely the battery, most parts stores would be happy to test your battery and charging system for free. 

I can 'almost' always tell when a battery is getting weak by the slighly slower-longer starts. A good parts store will have a load box or Midtronics device to test the battery in-car.

 

of course, there can be issues with corroded terminals and cables - but a well-maintaned 06 may be too young for those issues to be severe. Same for the starter.(though, my 06 WRX's starter drags/'is slow to release the drive gear',  in the cold - it still starts the car just fine)

i took all the cables off and replaced them with larger cables. it was probably corrosion but problem solved.