Compost/straw is not mulch. It's fertilizer. You need wood chips or bark or plastic products that won't biodegrade (messing up ph and chemistry).
Sorry I have to chime in here, I am just trying to help.
Actually compost/straw IS mulch -
Compost being fully broken down materials in which the nutrients in the compost will be readily available for a living plant to utilize. Compost also nourishes the Mycorrhiza and encourages them to flourish, helping your living plant do very well. It ALSO does what a mulch needs to do: Makes it easier to weed, minimizes temperature fluctuation in the soil, minimizes water evaporation in the soil. PH will settle, and if you are that concerned about 'PH' buy a test kit. Then you will need to figure out what PH hops like, and you can add certain things to your soil (IE Lime etc) to adjust that if you need to - this will be very rarely as hops are like weeds, they are comfortable in most PH situations. Not sure what you are trying to say about 'chemistry', unless you are planning on growing your hops in a sterile hydroponic system.
Straw being seedless dried hay that is not yet broken down into compost. It will compost, slowly, but accomplishes all the things a mulch needs to do... see above.
Wood chips/bark are much like straw except the do not break down as quickly. Plastic products - I fail to see why these are needed in any circumstance growing hops. Basically these are usefull for only the 'mulch' solution but they will not provide anything to your plants/soil - in summary a waste of money why not use compost you get 2 for 1. Unless you are growing your hops in a sterile soil using only chemical fertilizers and you do not want to provide healthy soil for Mycorrhiza, earthworms, and beneficial insect populations that help keep away pests. Also, your plants will be much more susceptible to disease vs if you were to use compost.
The best mulch I have used for any plant is compost, covered by shredded leaves OR straw. You will have the best soil imaginable in 2-3 years and your hops will be AWESOME.
Caveat - I am not a hippy and I do use chemical ferts sometimes - but I know the benefits of healthy soil and defend those benefits often.
To the OP - one thing I do successfully to keep the grass down is use lots of newspaper or cardboard and lay it on the area you want to kill them. This works well though is not very attractive looking. You need to weigh them down unless you live in a wet area.