It rather depends what you mean by 'wood mulch'. Bark chips are not the same thing as wood mulch, for one thing, and they are more commonly used as mulch. If its fresh wood mulch, which wood it came from is important - whether its been through a chipper and was once a tree, what variety of tree it was, whether fresh wood chippings have been composted for a few months first before use, and which plants they're going to be spread around, all these things make a difference. Fresh wood chippings (which are essentially cellulose) from a felled tree, if spread on planted ground, will, at the very least, deprive the plants growing in that area of nitrogen for a while, since the bacteria responsible for breaking down the wood require nitrogen to function, so they rob it from the soil, leaving a shortage for plants. This can be addressed by applying nitrogen before applying the wood chips though, and over time, the nitrogen deficiency corrects itself without intervention, as the wood chippings decompose, so on unplanted areas, it doesn't cause a problem. Wood chippings from treated woods should not be used for horticultural purposes.
Regarding whether wood chips are harmful to seeds, again, its down to the wood used, and the size of the chippings - smaller seeds may find it difficult to grow through a 3 inch thick layer of chunky, raw wood chips for one thing, but I wouldn't recommend wood chips being used where seeds have been planted.
On the subject of methane gas, a large pile of wood chips may well produce methane gas because it'll become anaerobic if its not turned regularly, and that leads to methane production. Regularly turning or stirring the pile will prevent that happening.
There's quite a lot of information in the link below - much of it applies to trees, and the information is American based, but the general principles are outlined, with some detail regarding various compositions of wood mulch and their possible negative/positive effects on plants. With regard to your salad garden, I wouldn't recommend wood mulch for an area like that - its best use is in areas planted with trees or shrubs, where regular digging and replanting doesn't take place.