Hello, is it possible to build a DVD with iDVD

All you need to do is open iDVD, select your theme and drag the movie file into the iDVD menu window, being careful to avoid any drop zones.

But how to control loudness?

Not sure what you mean by that but any device you play the resulting video DVD on can control the volume of the play back.

Follow this workflow to help assure the best qualty video DVD:

Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.

To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.

Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.