JAZZ Glenn Miller: ''In the Digital Mood.'' With the Glenn Miller Orchestra, conducted by Larry O'Brien. (GRP Records A-1002.) The idea of putting out this rehash of Glenn Miller hits was to bring top quality sound to these old classics (''String of Pearls,'' ''Moonlight Serenade,'' ''Pennsylvania 6-5000,'' ''Chattanooga Choo-Choo,'' and so forth), while being as faithful as possible to the originals. These remakes are not only faithful, they are virtual carbon copies, right down to the vocals, competently sung by Julius LaRosa, with backup by Marlene VerPlanck, Marty Nelson, and Michael Mark, and a whistled (!) solo by Mel Torme. Basically, this is a pickup band, not the group that conductor-trombonist Larry O'Brien is currently taking on a world tour. Apparently the idea was to have a few ''names,'' such as guitarist Bucky Pizzarelli and bassist Jay Leonhart, on the date. Aside from the excellent overall sound quality of the recording, one is tempted to ask ''Why bother?'' - since there are some fine reissues of the original Glenn Miller Orchestra available. True, the arrangements sound polished and the vocals are great, perhaps even better than the ones on the original Miller recordings. But there's something rather sterile about the whole business, and Jay Leonhart's bass, probably through no fault of his own, sounds too whiny and electronic to be convincing in this context. If this album will introduce Miller's music to people who otherwise wouldn't be acquainted with it, that's fine, but Miller fans are bound to prefer the real thing.