Guest Stoney has a good point. It is worth checking the angle of "break" of the strings over the bridge. As a general rule the shallower the quieter. If you have a very low action then raising bridge height will give a little more volume but at the expense of speed and ease of fretting and possibly also intonation.

You may have light gauge strings, e.g. .10-.34. If so, moving to something like .11 to .41 will give more volume. Personally I wouldn't go higher than that...particularly if you are coming back to the mando after a long break.

If it hasn't been used for a few years, the mando may not yet have "woken up" (a technical luthier's term!) and the sound may improve as you play it more (I could go into the physics and material science of this but haven't a full day to spare just now!).

I wouldn't go to too heavy a pick personally or you'll find triplets and the like become more difficult and the sound will be duller. Certainly not too much above medium gauge (I usually use a .60 Dunlop or .71 maximum). This is, however, a personal preference thing and also depends on your technique.

Ozark mandolins are not the loudest but you should be able to get a reasonable volume from it with a bit of perseverence. What others have said about it sounding louder to the listener is true as the majority of the sound is projected away from you.

What materials are the nut and bridge (with or without separate saddle) made from? Bone will give the best results for nut (unless there is a zero fret in which case it won't make a difference). For the bridge a good quality rosewood or ebony is preferable. I use a rosewood type with inset bone saddle. I buy them a little too high and then lower from the saddle side which then gives me the possibility to individually compensate each string.

Make sure also that the base of the bridge is in intimate contact with the top of the instrument along the areas that are in contact. Gaps here will result in a loss of transmission and to the top and less vibration of the soundboard. The bridge must be properly "fitted" to the top, especially if there is any curvature in the latter, rather like a fiddle bridge must be properly seated. This is a job you can do yourself (PM me if you need details of how) or any competent fretted instrument repair man should be able to help.

Hope these random ramblings help a little.

Peace

mooman