If you find bubbles or specks of some kind under new wallpaper, first cut an X pattern over the defect. If the problem is a speck, remove it with a tweezers or a utility knife. Then work seam adhesive into the cavity, squeeze out the excess, and seal the opening. For air bubbles you also can use a paste-injecting syringe found in paint and wallpaper departments.
There’s more to soundproofing than just insulation. “Blocking” noise with a non-hardening caulk to close off airborne sound paths. “Breaking” the sound path using resilient wood studs with metal clips. Last, isolating and eliminating “vibration” with special acoustical matting that seperates the wall structure from floor vibrations. You can get all these items in soundproofing kits for both interior and exterior residential walls. Besides blocking most noise transmission, one obvious benefit is better energy efficiency, with greater “r” values.