The MG soil has just become anaerobic sitting around in a bag holding moisture - that's what accounts for the smell. But there's nothing wrong with it other than being waterlogged and it can safely be used as a mulch. Or you can just allow it to dry out and use it as a short term potting soil.

Straw is often used as a mulch but can often contain seeds that will sprout, turning your mulched bed into a wheat field.

Straw bales exposed to weather can develop toxic molds inside just as in hay bales do. If, when you break that bale open, you find a white mold due care should be practiced in further handling. Straw often has seeds of what it grew that do germinate and grow, giving you more of what is in that straw bale, the wheat, oats, rye, barley, etc. or what could now be a green manure or cover crop.
Moisture, in large enough quantities, can displace the air in soil, or soilless mixes, which can then allow anaeorbic (absence of air) bacteria to develop. Those anaerobic bacteria can produce unpleasant odors. Spread that mix out so it can dry some which will stay those anaerobic bacteria.

OK! Thank you both. I'll use the soil as mulch and will pass the straw into our greenwaste recycling.

"you know those small, decorative straw bales they sell in the fall?"

Unless I'm missing something here, I wouldn't toss the straw.

Has it been dyed or anything? If not, straw comes from annuals, and it shouldn't cause a weed problem from using it as mulch.

A few leftover seeds may sprout but be easy to pull. If you don't let them go to seed they will be gone and you have added good stuff to your soil!

Whether straw comes from an annual or perennial cereal grain has little bearing on how seedy or weedy it may be. Seeds can be viable long after the grain is harvested and if the straw has not be thoroughly cleaned of seeds (and it seldom ever is), there remains the potential for those seeds to sprout.

I work as a horticultural consultant and I can't count the number of times folks have come to me with this problem. Using the straw overwinter as a mulch is one thing - too cold for the seeds to germinate. Using the same product during the growing season is another matter altogether, unless you are very much on top of the weeding chores. And most folks mulch to avoid weeding.

I've used dozens of bales of hay/straw(?) as a mulch in my garden for years and I guess I've been very lucky to never have a problem with seeds growing. After pulling up the dead veggie plants I leave the hay/straw to rot over the winter and reapply with whatever bales I can find or get cheap when I start the garden each spring.

So do you think I'm just lucky or is it that because I keep putting 6 inches of it down each year that that keeps the seeds from sprouting?
I don't want to tempt fate but I would like to know if others have been "lucky" to be weed free also with random bales of straw and hay?
Thanks